You gotta have cash even in the future, otherwise you can't buy anything. Utopian socialist paradises excluded of course.
Traditionally, the unit of currency in science fictional futures is called the "credit". This is the futuristic equivalent of a dollar, Euro, or whatever. Using metric, one megacredit is a cool million credits. One kilocredit is a grand.
And there are a few examples of science fictional socialist utopias where money is obsolete, e.g., Star Trek. Good luck with that.
- In SPI's RPG Universe and Star Trader, the unit of currency was the "Tran" or "transaction", where 1 Tran was equal to about $500.
- SPI's Star Force had "LaborCredits".
- In Karl Gallagher's Torchship the unit of currency is called Keynes or "Keys", named after Keynesian Economics.
- In Philip E. High's The Prodigal Sun money was literally hours of work.
- EVE Online has a little more complex a take on things. The currency, known as ISK (Inter-Stellar Kredits), is not so much a global currency as it is a global exchange currency. Planetary economies and sometimes individual planetary nations almost all have their own currencies, ISK was merely setup as an exchange medium to manage the obscene amounts of money being used at the interstellar level
- In The Great Explosion by Eric Frank Russell, the planet K22g is a post-money utopian society, but they still have a medium of exchange. They use favor-exchange based on "obs" (obligations). This might explain the value of the poker chips you see in all those Star Trek poker games.
- In John Morressy's Del Whitby series, the unit of currency was the cash-cube. These were cubical coins of precious metal which would stack into neat rectilinear piles.
- In Diane Duane's My Enemy, My Ally, the Romulan's currency is in the form of chains of precious metal.
- In John Brunner's Intersellar Empire series, the currency is in the form of rings of preciouis metal.
- In Frank Herbert's novel DUNE, the Fremen's currency is based on liters of water, symbolized by metal rings. They tie the rings in strips of cloth so as to not make noise when they are sneaking up on an enemy.
- In the simulation game High Frontier, the unit of currency is "the most valuable thing in the universe", namely water. Water can be used for reaction mass, as a source of hydrogen and oxygen, radiation shielding, and a host of other uses. The unit is a 40 metric ton tank.
- In Kim Stanley Robinson's Red Mars Trilogy, calories of heat were used as the basis of the Martian economy.
- I am somewhat dubious about the Quasi Universal Intergalactic Denomination. Apparently it is intended to be safe in the space environment and will survive the space environment. This means it is constructed out of a space-qualified polymer, emit no toxic fumes, has no sharp edged, be resistant to high temperatures, and not use a magnetic strip like a credit card since cosmic radiation will render them inoperative.
- Of course nowadays most people use credit cards and PayPal.
The following currencies are perilously close to being trade tokens, since they are issued by a corporation instead of the government.
- In Andy Weir's novel Artemis, the lunar colony is officially Kenya Offshore Platform Artemis, property of the Kenya Space Corporation (KSC). The unit of currency is the KSC "Slug" (symbol "ğ" or g with breve). This is slang for "soft-landed-gram" or SLG. One slug is the price KSC charges to deliver one gram of payload from Kenya spaceport to a soft landing at Artemis colony, Luna. They are more like a pre-purchased credit service from KSC. Since Artemis colony is not a nation as such, they use slugs for money.
From a practical standpoint, this makes KSC a bank and the slug a useful medium for criminals to do money laundering.
In the novel the current (about 2070 CE) exchange rate is $1 US = 6ğ or 17¢ US = 1ğ.
This imples a payload delivery price of $170 US (2070 CE) per kilogram, which is towards the optimistic side.
- In the Micronauts series of comic books, the evil Baron Karza has a monopoly on lifespan prolongation technology (the "Body Banks"). He issues his own currency called "Life Credits", with which a person can purchase extended lifespan. The underclass waste all their credits in gambling institutions, and can sell personal organs and other body parts in exchange for more life credits. The aristocracy is firmly under control of Karza, since they know he literally has the power of life or death over them.
- In the Battletech universe, a common unit of currency was the C-Bill, redeemable for a certain amount of data transmission on Comstar's FTL communications network. Other noble houses issued their own currency, but the relative worth fluctuated wildly with the fortunes of the issuing house. Mercenaries prefered to be paid in C-Bills because it was relatively stable. 1 C-Bill (3025 CE) = $5 US (1986 CE).
- In Damon Knight's Idiot Stick, aliens arrive and want to hire humans to do some construction labor using alien tools. They are paid in "Happy Caps", which look like plastic soap bubbles. When you touch one, the charge enters you and you experience a few seconds of intense happiness. The happy cap loses its charge in the process and becomes worthless. Entrepreneurs in near-by New York start marking prices not only in dollars ($), but in HappyCaps (Hc).
Money has four functions:
- Medium Of Exchange: a way to avoid the double coincidence of wants problem inherent in the barter system. Or "he wants two coconuts for that shirt but all I got is this mango."
- Common Measure Of Value (or Unit Of Account): a way to set a standard price tag on market value of goods, services, and other transactions. This is essential to allow commercial agreements that involve debt. The loan officer ain't gonna give you a loan for that used car if you are using the barter system. It is too hard to figure the car's value in tiger-skins.
- Standard Of Value (or Standard Of Deferred Payment): using money to settle a debt sometime in the future. As Wimpey says "I'll gladly pay you Tuesday for a hamburger today"
- Store Of Value: So you can bank your money, with the expectation that when you withdraw the money it is still worth something. This doesn't work with the Banana Standard, put a banana in a bank box and you will be dismayed to find in a few years it has turned into worthless moldy goo.
Note that while Planet A and Planet B may both internally use a barter system instead of money, they might use arbitrary money (a nonstandard medium of exchange) when trading between each other. Using a medium of exchange avoids the problem of ensuring there is a double coincidence of wants, which is a problem inherent in the barter system.
The point is that a double coincidence of wants does not happen very often, which makes the barter system rather awkward and impractical. If however you create some arbitrary money-like medium of exchange, you can avoid the whole mess. It also avoids the problem of when one of the things you are bartering has a shelf-life or is seasonal.
TRADE TOKENS: Sometimes you will see traders using Trade Tokens. These are basically money that is not issued by a government, but instead by a private company, group, association or individual. From the 17th to the early 19th century these were used by merchants because the local government was not up to the task of issuing enough coins to allow business to operate. Nowadays you generally see them in the form of casino chips, in video arcades, and car washes. But a hypothetical interstellar trading company might issue their own trade tokens if there were no local government in the trade area, or at least one single government recognized by all the trade planets.
SCRIP: And Scrip is an even more localized form of trade tokens. You often see this in old time mining or logging camps. The employees were not paid in money, but instead in company scrip. The company scrip could only be spent in the company store. Due to this Truck system, the employees more often than not wound up owing their soul to the company store.
COINS: Historically coins were disc shaped because that's what you get when you roll out a measured ball of precious metal and flatten it with a stamp bearing the King's face (the precious metal is called "specie"). They are also convenient to carry, especially if they have a hole in the middle for stringing. You should mill the edges to prevent crooks from engaging in the crime of coin clipping (this does little to stop the crime of "sweating" coins). Though nowadays most US coins are composed of such worthless metal that the milling is purely decorative or as a aid to the visually handicapped.
Gold coins are generally composed of a gold alloy (Coinage Gold) because pure gold is too soft to hold up to the stress of being used as a coin (and makes it too easy for criminals to clip or sweat the coins).
In modern times, a popular popular choice for silver-colored coins is the alloy cupronickel, due to its corrosion resistance, electrical conductivity, durability, malleability, low allergy risk, ease of stamping (metalworking), antimicrobial properties and recyclability. Cupronickel is not for commodity money, the alloy is relatively valueless.
In low-tech places, coins are manufactured by hammering or casting. In more modern-day tech settings, coins are machine-struck. In futuristic data-driven societies, physical coins are obsolete and instead virtual coins are used. Indeed, as previously mentioned, physical money may actually be illegal since the Police State cannot trace physical money transactions.
In medieval times there were so many currencies that merchants had to carry coin pan balances in order to determine the worth of a given coin.
There are three basic monetary systems:
- Commodity money: The currency is physical lumps of something valuable, e.g., solid coinage gold coins. Sometimes the coins have Seigniorage, meaning when you purchase the commodity money at the treasury the cost is a bit more than the mere market rate of the precious metal (see link for details).
- Representative or Commodity-backed money: The currency is paper bills or worthless coins that are backed by stores of something valuable in the treasury. This is sometimes called the Specie Standard. Example: the Gold standard. Paper gold certificates can only be printed by the treasury up to the amount of gold in their vaults. Most nations on Terra abandoned the gold standard during the 20th century due to the inherent problems. Most current economists are not in favor of a return to the gold standard, and all efforts in the US to legislate such a return have failed. But the Gold Bugs never stop trying.
- Fiat money: when the paper bills have value because the issuing government says so. Pretty much all money in the real world is fiat money. Most modern governments use it because it allows flexibility during a financial crisis. It allows the government to adopt an expansionary monetary policy aka "printing money".
Unless all the planets you trade with are members of the same interstellar government, or there exists some sort of interstellar money-changing organization, the money used on one planet is worthless on another planet.
If you have a group of planets that share a common currency, for the planets sake it is vitally important that they share a common fiscal policy. The ongoing Eurozone crisis has been made much worse by the fact that while the Eurozone has monetary union (i.e., one currency, the Euro) it does not have fiscal union (e.g., different tax and public pension rules). This ties the hands of European leaders, making the crisis almost impossible to solve. When the Eurozone was proposed, the various nations were persuaded to surrender their currency, but reluctant to surrender control of their fiscal policy (give up their national sovereignty? Never!). The proponents figured to get around the problem by doing the Eurozone union in two stages, which in retrospect was an insanely bad decision.
Examples of Commodity money in science fiction include:
- HEX COMIC BOOK: In the post-apocalyptic world, the currency are Soames pills used to decontaminate radioactive water.
- TANK GIRL COMIC BOOK: currency is actual bottle of potable water. No symbolic money rings as is in DUNE.
- MAD MAX 2: currency are jerrycans full of automobile gasoline.
- IN TIME: currency is actually time from one's lifespan.
- STAR WARS: THE FORCE AWAKENS: on the scavenger world Jakku, the junk dealers pay for salvaged stuff with "portions", some sort of cheap food.
- DINNER AT DEVIANT'S PALACE by Tim Powers: in post-apocalyptic California, the currency is bottles of alcohol. A fuel, disinfectant, and a beverage all in one.
- CHILDE CYCLE by Gordon Dickson: the interstellar currency between planetary governments is skilled professionals. The economy of the planets is based on the trade of contracts of specialists.
- JAILHOUSE ROCK: in the prison the richest man is Elvis Presley's mentor, who has hundreds of cartons of cigarettes.
- DEADLANDS HELL ON EARTH GAME: In the post-apocalyptic world, bullets are hard currency due to high demand and low or non-existent production.
- GURPS AFTER THE END GAME: In the post-apocalyptic world, rifle cartridges are currency
- STARGATE SG-1: galactic trade is conducted with "naquadah", the unobtanium mineral that magically amplifies energy sources.
- METRO 2033 GAME: In the post-apocalyptic world, pre-apocalyptic military-grade bullets are currency, since the new bullets being currently manufactured are pretty pathetic in comparision.
- CITIES IN FLIGHT SERIES by James Blish: the interstellar economy uses the OC Dollar which is on the Germanium Standard (basis for semiconductor electronic chips). Until some moron invents a way to synthesize germanium and crashes the economy of the entire freaking galaxy. They try to put it on the Anti-agathic Drug standard but that doesn't work very well because people have to use their drugs or lose immortality.
- REAL-WORLD COLONIAL VIRGINIA: tobacco was currency.
- REAL-WORLD RUSSIA: in the nineties and currently in remote areas, bottles of vodka are currency.
- ANCIENT ROME: soldiers were partially paid with salt.
Examples of Commodity-Backed money in science fiction include:
- THE PRODIGAL SUN by Philip E. High: money backed by hours of work. As a person works, their hours of labor are accumulated as pay-hours. Basic labor has a factor of 1, where one hour of labor earns 1 pay-hour. More skilled has higher factors: a reporter has a factor of 6 or one hour of labor earns 6 pay hours. Goods and services have purchase prices in terms of pay hours.
- STAR FORCE GAME by SPI: the currency is called LaborCredits, which from the name is probably more or less the same as in The Prodigal Sun.
- THE HIGH FRONTIER BOARDGAME: the economy of industrializing the solar system is backed by tanks of water (the most useful substance in the universe). 1 tank holds 40 metric tons of water
- THE RESTAURANT AT THE END OF THE UNIVERSE by Douglas Adams: When the Golgafrinchan space ark B dumps its moron colonists on the primitive Planet Earth, they adopt the tree leaf as legal tender. This makes them all rich, in theory. Unfortunately due to the abundant supply of leaves there is rampant inflation. You need three deciduous forests to purchase one ship’s peanut. To deal with the problem they embark on a program to revalue the leaf by burning down all the forests. Things rapidly go downhill from there.
- CONFEDERATION UNIVERSE by Peter Hamilton: money is backed by Helium-3, sine qua non of D-3He fusion. The fuseodollar is on the He3 Standard, since helium-3 is somewhat rare.
- TIME ENOUGH FOR LOVE by Robert Heinlein: on the colony on planet New Beginnings the protagonist sets up a bank since nobody else knows enough about finance to do it. For practical reasons the money is backed by seed wheat, which not good since it is perishable but there isn't enough gold or silver around to support the economy. The protagonist pegged the money to seed wheat in order to stabilize wheat prices, which the colony needs in order to grow.
- BLOOM by Wil McCarthy: the currency is backed by uranium with the basic unit being one gram. This is not due to its use in fission, but rather in the transmutation potential of ladderdown reactions. On this scale the most worthless element is Iron-56.
- DUNE by Frank Herbert: the currency is backed by potable water on the desert planet. Underground cisterns "bank" the water, when a person adds water they are given currency in the form of rings.
- BATTLETECH GAMES: In this universe the ComStar megacorporation has more or less a monopoly on faster-than-light communication after the collapse of the Star League. ComStar used their Letter of Credit to bill the various Great Houses for usage of the ComStar network. The Letter of Credit was intended as a method of payment for those services, its value based on a fixed amount of ComStar service, transmission time, or delivery distance.
Now each of the Great Houses had their own individual currencies, called House Bills. The exchange rate of H-bills between two different houses would wildly fluctuate. Mercenaries (who operated between various houses more than inside a given house) got the bright idea of requesting payment in ComStar Letters of Credit to ensure a consistent payment basis.
This became the "C-bill" where 1 C-bill = 1 second of text-only FTL transmission time. So it is a trans-empire common currency backed by seconds of FTL transmission.
- RGB MARS TRILOGY by Kim Stanley Robinson: the currency is backed by calories of heat.
- WARLOCK IN SPITE OF HIMSELF SERIES by Christopher Stasheff: the interstellar government's currency is the "kwaher" or currency backed by kilowatt-hours of energy.
- THE SHAPE OF THINGS TO COME by H. G. Wells: the World Transport Union (WTU) creates the "air dollar." 1 air dollar will purchase the transport of one kilogram the distance of one kilometer on board a WTU aircraft.
- REAL-WORLD COLONIAL VIRGINIA: certificates issued for delivering tobacco to warehouses was the first United States commodity-backed money.
Debasing a currency means making a nation's money worth less. Generally this is done by the nation in question in order to gain some financial advantage at the expense of its citizens. The historic method was to secretly reduce the precious metal content of each coin, assuming a government unsophisticated enough to actually think that the gold standard is a good idea. If the goverment is not on the gold or other valuable element standard, it can engage in the more modern method of currency debasement by printing more paper money.
Soon Gresham's law rears its ugly head. The old more valuable coins will vanish from circulation as people horde them, and the new less valuable coins will become widely used.
Obviously criminals will engage in do-it-yourself unofficial currency debasement, aka "counterfeiting". If the local government is on the gold standard the crooks will clip the coins, basically reducing the gold content of the coin by making the coin smaller, and melting the little trimmed or filed off bits into ingots. If the govenment uses paper money it is time for the eternal war between crooks cranking out counterfeit money with commercially available printing technology (paper and 3D) and governments trying more and more high-tech anti-counterfeiting measures. Counterfeiting is always illegal, and the penalties are severe. The government is of the opinion that it is the only one authorized to debase the currency, and it is real touchy about unfair competition.
And some reckless criminals will engage in the insanely dangerous crime of counterfeiting those trade tokens known as "casino tokens". This is outrageously risky for two reasons:  the tokens have anti-counterfeiting measures and counterfeit detection technology orders of magnitude more sophisticated that government measures, and  since many casinos are owned and operated by organized crime they will simply murder you instead of bothering to call the cops.
In science fiction debasement sometimes happens for more radical reasons. Some science fictional means suddenly destroys the value of the material used for the standard; e.g., some idiot invents a device that cranks out tons of gold, thus blowing the gold standard out of the water and crashing the entire economy. Alternatively another idiot invents some new and improved method of counterfeiting money, commonly by inventing a matter-duplicator which can make perfect duplicates indistinguishable from the original.
- In James Blish's Cities in Flight the OC Dollar was based on the Germanium Standard, because of its vital use in transistors and computer chips. This worked fine, until some joker figured out how to synthesise germanium and thus obliterated the economy of the whole galaxy.
- In George O, Smith's "Pandora's Millions", the invention of a replicator crashed the economy of the solar system. Replicators mean there are no longer any rare metals to base your money on, and all material goods become basically free. The only thing of value are personal services (such as those of a surgeon or doctor). The only thing that prevents utter disaster is a synthetic element that cannot be replicated (because replication causes it to explode). The element allows one to create cheques, legal tender, and other critical items that cannot be counterfeited by a replicator.
- In the Star Trek universe, the Federation is a post-money society that uses replicators. The Ferengi use "gold-pressed Latinum" as the basis of their currency, since Latinum is the one element a replicator cannot create.
- In the Demon Princes pentology by Jack Vance the currency 'SVU' or Standard Value Unit was a printed note equal in value to one hour of common labor. A device called a "fake meter" is used to detect counterfeits. In the second novel the protagonist discover how to fool the fake meter, and hilarity ensues.
In some science fiction, physical money is illegal since the Police State cannot trace physical money transactions (see Cashless Society). By law all money transations must be via credit cards or debit cards. Intangible electronic numbers transferred from one bank ledger to another through traceable internet connections (credit cards or debit cards), so the state can keep a close eye on all your purchases. To the very last penny.
Naturally people wanting to bribe somebody or purchase illegal goods had to find some physical untraceable alternative. Purchasing illegal drugs with your bank card will just get you busted by the narks.
Such illegal currency is related to the concept of money laundering. But it is also required for disadvantaged people who for whatever reasons cannot obtain a credit or debit card. They literally cannot be paid or purchase anything. Not to mention people who do have such cards but have the cards or their identities stolen. Until new cards can be issued the victims with stolen cards also cannot be paid or purchase anything, which can be a week or so.
Keep in mind that the definition of "illegal" can abruptly change if the government takes an unexpected turn for the totalitarian.
In 1940s Germany, it suddenly became the law that people of Hebraic heritage were to be rounded up and sent to the death camps. Some compassionate people would hide Hebraic families in places such as their house attics. This would be much harder if there was no anonymous money.
If the government can see every single purchase you make via government mandated credit cards, it will be easy for them to spot that you are purchasing more groceries than is reasonable for a family of your size. Next thing you know the secret police is pounding on your door, wanting to inspect your attic. This is analogous to how narcotic police spot underground cannabis farms by abnormally high electricity bills, for all the grow-lights.
In underworld crime thriller novels, shadowy characters sometimes use Krugerrand gold coins to purchase illegal goods on the theory that Krugerrands are untraceable.
In the science fiction show The Expanse, Detective Miller accepts bribes in the form of casino chips. One of the authors said: "Why do they use casino chips as money on The Expanse? It's how you get a black market economy when everything else is traceable e-money."
In the real world, events overtook science fiction when some genius invented a way to transfer money over the internet, but encrypted in such a way that the authorities could not track it. Specifically they can see the transaction but have no idea of the identity of the sender or receiver. This is called Bitcoin (BTC), symbolized by , Ƀ, or ฿. Bitcoin is the first cryptocurrency that is decentralized.
There are those who are of the opinion that anybody who uses Bitcoins, especially for investment speculation, is an idiot. They use a disparaging term for Bitcoin, calling them "Dunning - Krugerrands"