Dirty money is an artifact. It has been abandoned by some hero who has since long forgotten about it. She had earned it just in time through waitressing at the local pub. She was on the way to pay the candle bill in person, but the money fell out through the hole in her backpack on the way. She even retraced her steps several times to see if she could find it, but the tears she cried out of desperation and frustration kept her from seeing what was right in front of her the whole time.
WHATEVER! It's yours now! Good thing your hero counts it first and asks questions later. Otherwise, they would be finding a sink to try to clean it off. The shopkeepers don't seem to mind, and still bite it to be sure it's real and sometimes comment regarding the flavor embued by whatever it is that is on these coins.