I have a box of foreign money at home which I use every time I am abroad. I bet most people have something like it.
If I were to trust you on this, I'd have to record the price of that currency with every cup of coffee in every airport I buy. In practice nobody has done that, ever. I'd say that's safe.
Should I suddenly pay cash for new sports cars or houses out of my little box that would be different of course. I doubt the difference between a cup of coffee and a sports car is really encoded in law, but the difference is there and should you find yourself in a grey area (old used car perhaps?) there are people specializing in this very matter that can help.