Currency is whatever we define it to be. It’s only “invalid” if one party attempts to use something other than what both parties have agreed to. If the shopkeeper accepts a pint of goat’s blood I give him in exchange for his goods, that’s not fraud. That’s an agreement between him and me. If he chooses to accept a Canadian Nickel in exchange for goods, that would be a similar agreement. That being said, if the shop keeper does raise an objection to it, then it’s my responsibility to provide currency that he does accept, or else void the sale.