Here in 2019 the overwhelming majority of all currency is virtual and commerce on any appreciable scale occurs electronically. But consider a sci-fi/space opera setting where reasonably fast FTL is commonplace, but FTL communications are not possible. Obviously one could still "communicate" at FTL with a courier, but you would still be limited to the speed of the courier ship. You certainly wouldn't have instantaneous communication between star systems, meaning there can be no interstellar electronic banking: transactions would take years to complete.

The Traveller tabletop RPG uses exactly this setup: FTL travel is common, FTL communication does not exist. In Traveller you have the Third Imperium minting currency that is accepted essentially everywhere, the currency is Imperial Credits and they're printed on polymer bills. The result is an effectively cash-only economy.

But what if your setting has no centralized government? Do people revert to using gold? Are there fleets of merchant ships schlepping precious metals around the cosmos, as if the American Old West has been transplanted into space? Would they come up with a cryptographic solution? Could something like a blockchain work without instantaneous communication cross the entire network that accepts the cryptocurrency?

What if quantum computing is widespread in your setting, rendering most forms of encryption obsolete? That would seem to eliminate the blockchain based option, FTL comms or not, and once again send us back to needing a fiat currency, or a gold standard.