What does the twenty dollars off slender level do

It does nothing at all. It is only another way to know Slender man is near you. You still die by slender man the same way as normal, but in the background you hear give me 20 dollars.
Do not go up to Slender man and give him 20 dollars. You will only lose your 20 dollars and life, and the game.

The "Give Me Twenty Dollars" is in fact a reference to Ron Brownz, who made that song. One of the creators of Slender happens to like Ron Brownz and as a joke, decided to add that song in.

You do not hear the music throughout the game, you only hear it when Slender is close to you.