Mr. Krabs ' Wacky Bucks is fake money that only appeared in the episode One Coarse Meal. Mr. Krabs pays SpongeBob with Wacky Bucks. In that episode, SpongeBob tells Pearl to buy something with them, but Pearl notices that the money's not real money.
Wacky Bucks look like the US-dollar. The differences is that Mr. Krabs is on it as the President, the Krusty Krab is in the background, it says "Not Legal Tender," and has Wacky Bucks is written on it.