Wacky Bucks are a made-up form of currency that Mr. Krabs pays SpongeBob whenever he feels cheap or cannot afford to pay him his weekly paycheck. They only appear in the episode "One Coarse Meal."
Wacky Bucks look like the US dollar. The differences are that Mr. Krabs is on them as the President, the Krusty Krab is in the background, they say "Not Legal Tender" at the bottom, have their name written on them, and have clams on them. They also have the number one on all corners.
Role in episode
When SpongeBob gives Pearl some money, the latter says "Hey, this isn't real money," prompting SpongeBob to reply "No, it's even better. It's the money Mr. Krabs pays me with: Mr. Krabs' Wacky Bucks." Stunned, Krabs says, "Hmm, the boy is catching up to me."