Exactly. You've got limited time to tell your story in a movie, so establishing a fake currency that nobody except hardcore numistatics nerds will care about is kinda pointless when everyone already understands that credits = future money.

And as you say using a real currency doesn't make sense in most future movie contexts, and using the "wrong" one may even offend some people who may then boycott your movie - which is obviously not desirable when you're a studio trying to make money.

I image its the same reason for the Gold/Silver currency in many fantasy settings.

I think in Rick and Morty one of the fake currencies is called a schmeckle, which is Yiddish for penis (diminutive).

The money can’t be any real earth currency

no, now that you've said this i want to see a sci-fi movie where all trade is based on the Vietnamese Đồng

No, they won't.

Credits are for future and guns, it's coins/pieces of gold/silver when swords.

what if their dollars are worms?

I think that's super valid for a novel especially, as long as you don't mind that the movie adaptation decides it's not worth taking 10 minutes out of the runtime to explain vs something people will immediately understand

Or maybe there is nothing else interesting in the novel, and it won't matter anyway because no one reads or adapts it