If there is someone looking for a $20, then give it back. But if there is no particular owner in sight, take it. Why not? Nobody is looking for it. Get yourself something, a pizza, chinese food . . . Any food : D But , yes. Take it .
If you see 20$, you should not take it. It is wrong to take any money that you find because it is morally wrong. If you happen to find 20$, you should turn it in to lost and found or to the person who it belongs to. 20$ is not worth breaking your morals for.