This reminds me of a brainstorm I once had: negative cash. You finish your meal at a restaurant, the waiter hands you the bill along with negative cash for the exact amount. You overwithheld your taxes this year, you file your return with a negative check for the amount of the refund. Beware, though, of criminals who sneak up behind you in a public place and drop negative cash in your pocket: they’re called putpockets.