Wacky Bucks are a made-up form of currency that Mr. Krabs pays SpongeBob with whenever he feels cheap or cannot afford to pay him his weekly paycheck. They appear in the episode "One Coarse Meal."
Wacky Bucks look like the US dollar. The only 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, she points out that the Wacky Bucks are not real currency, prompting SpongeBob to state that it is "even better" and the money that Mr. Krabs pays him with. Stunned, Krabs says, "Hmm, the boy is catching up to me."
The numbers on the bottom of the bills read "01123581321". This is the start of the fibonacci sequence, a sequence in which the next number in the sequence is the sum of the previous two.
In spite of Mr. Krabs paying SpongeBob with these, we never see them in any other episodes.