Currently this is kinda already the case. The USD is backed by oil. If you have $100 (give or take) you can exchange it for a barrel of oil, and vice versa. Now the USD is only loosely pegged to the price of oil, (The price is adjusted by things like: sell military equipment cheap for a few favours on production output, release some reserves when prices get a bit high, go to war with any smaller countries that try sell oil in euros, that kind of thing)
A good currency needs to be:
- Divisible (so you can get change)
- Have some sort of backing
- Is reasonably stable price
As a side note Bitcoin does really well at 1&2, but poorly on 3 (a combo of it's utility of 1&2 + the size of the Bitcoin economy (the amount transferred daily which currently is much smaller than the whole pool of coins)) And point 4, Bitcoin is currently too volatile to be a useful currency
The USD used to have gold as a backing. But because of the French gaming the system President Nixon changed it to be backed by the US Government, or effectively the US economy. Which for reasons also includes almost all of the International oil trade.
So for your world building, raw material commodity prices fluxuate wildly; as astroids get brought into the Luna dockyards, some worth dozens of credits, others just piles of useless gold and gravel. However the lowest common denominator is always energy, antimatter to be precise.
Antimatter is divisible (you wouldn't want to fuel up a ship with more than 100 milli (grams) now would you.
Antimatter is Transferable. One gram (one credit worth) is easy enough to move from (person) A to B; even in it's containment vessel. FWIW one gram is enough energy to be equlivent to that released on Hiroshima and Nagasaki by the US in WWII
It is backed by it's own value and utility.
Any ship larger than a fusion powered rock-hopper is going to need antimatter. In the same way virtually all of present day's transportation (air, train, plane, automobile, truck) uses oil.
Creating antimatter requires a Huge investment in cyclotron infrastructure. The "Corporation" was the first and only entity to build a luna antimatter factory and basically captured and controls the market.
After some big price fluxuations in gold, titanium, water etc., as the first antimatter delivered astroids started coming in, a lot of mining outfits started going bust because they could not afford to refuel their ships. The "Corporation" started issusing "Antimatter Credit future's ", which can be exchanged for antimatter, on delivery of an astroid; irrespective of the current commodity spot price. Before long Credits became the defacto currenency of the off world economy.
Because Credits can be exchanged for something of fixed value (1g of antimatter), which has great utility, and the whole space economy runs on it; its value is very stable. Plus the future's contracts, helped even out the price fluxuations of astroid derived commodities.
One Credit is worth a fortune so when someone refers to a credit ( credit with lower case c or ¢) what they really mean is a micro credit, one millionth of a credit. Change is given in mil or milli, one 1000th of a small ¢redit. For perspective 2 mil will buy a can of cola.