If the singular most important currency is issued by an institution and everyone uses it, then they would have to have immense power. It cant just be the leaders, with whom the player interacts. Assuming enemies drop currency (why would they use the same currency like a hostile entity)

If there is no such institution, then I would make the currency gold or another slightly valuable resource, that is common enough that it realistically can be made into currency affordably. The gold route is probably better if you want 1 currency for all tribes/nations to use (player convenience)

We are survivors of an apocalypse that hit the world of Ghost. I am not sure how big the surviving cities/civilizations are. Our value of currency exists because we believe it to have value. Does an organisation exist in ghost that makes us believe so. If yes, a cringe name could work.