Okay, so usually in fantasy RPGs the currency is generic gold, silver, and copper coins. Occasionally you'll find games wherein the local authorities will insist that "foreign" coins be exchanged for locally minted ones. This is usually presented as a straight up racket to force foreigners to pay a percentage on coinage brought into the area (and a racket by the DM to keep the PCs from getting too rich). There could be various other reasons, such as foreign coins being different sizes and thus having different values or coins from a certain region known for being debased with other metals. But recently I came across this interesting bit on Wikipedia from the story of Jesus driving the money-changers from the temple:
"Gentile money could not be used at the Temple because of the graven
images on it."
Hmm, that's a much more interesting idea. Imagine our heroes score a hoard of coins from the evil cult temple dungeon. Hooray! But when they get back to civilization the locals refuse to take any of it because it has the symbol of an evil deity on it, which our heroes failed to notice in their excitement. Maybe they are forced to melt it all down to destroy the images, perhaps into plain bars, costing them some cash. Maybe priestly types from the temples of good deities insist that it undergo cleansing rituals to make sure it's not cursed or otherwise tainted. Weird coinage can be turned into interesting role play situations.