It looks just like any other ordinary dollar, but it has a lot of sentimental value to Mr. Krabs.
Role in episode
After Sandals buys coral bits, Mr. Krabs runs out of his office and announces that he just gave him his millionth dollar. After kicking the customers out, Mr. Krabs decides to take SpongeBob and Squidward on a fishing trip with his millionth dollar. On the ship, SpongeBob plays with his fishing rod and ends up hooking the dollar in the ocean. Soon dramatic music plays and SpongeBob frantically reels it back on the ship. However, the dollar is eaten by a clam when it is brought back on the ship and Mr. Krabs makes SpongeBob and Squidward stay clam fishing until they find his millionth dollar.
At the end of the episode, Mr. Krabs trades his body for his millionth dollar with the clam that took his dollar.