Outside of a specific faction backing one, there will not be a universal currency, there will just be barter with various items having more or less value to different individuals. The only thing “old-world” money will be good for is either starting fires or buying things from robots.
For example if you’re trading with a hermit that’s hiding in a cabin in the middle of the woods, they have no interest in pre-apocalyptic or faction currency, they have no intention of going to a town where they can get things from a robot, and they’re too far from a faction to use their fiat currency, all they’ll be interested in are things they can use to survive in the immediate future.
If on the other hand you’re trading with a faction merchant, they might accept their faction scrip as currency, but nothing else, depending on diplomatic state with other factions they might even accept scrip from other factions. If you don’t have that though you can still sell them things the faction needs or barter.
A roving trader might accept lots of different faction currencies, because carry weight/volume is at a large premium, and incidentally their valuation of other items would be heavily biased by weight/volume too.
In other words, it’s situational, there should be no universals.