[sammyg2]

$20 can be very little money, or it can be a great deal of money.
If you keep it and don't try to give it back, then that $20 just cost you your self respect and that is expensive.
if you try and find the owner, you get to look yourself in the mirror and see a person of morals, a good person. In that case that $20 was cleap compared to the reward.

Post a note that says "found, a certain amount of cash".
If someone responds and says "I lost a $20 bill" give it to them.
If no one can describe exactly how much was lost and what the denomination of the currency was, then you get to keep it with a clear conscience.