Also, depending on where and when in the middle ages, currency would vary hugely and work with several dozens of different coins. Travel to another region would involve money changers, scales, entirely different systems of coinage with (by modern standards) absurd breakdowns of coins and wholly new names and words.

And having pure coins would be easy, so obviously you'd have fun stuff like "new dollars" being worth 7/16th of an "old dollar" because they have less silver in them.