Maestro
Master Sergeant
Your $20? Or someone elses? Karma. Believe it, B. Turn it in. If it's not claimed (surely you didn't brag about it, right buddy?), likely the school will give it to you after a period of time. To do otherwise is theft.
... Or the person you turned it to will put it in his pockets. Believe me, when you find money (that isn't in a wallet, of course), you're better to keep it for yourself. After all, getting yourself richer is better that getting an other @sshole richer.
Of course, if the money is in a wallet, the above statement can't be taken into consideration. That is where I draw the line... If it is in a wallet (or a suitcase or whatever), then the owner will surely want it back, so you better play the "cool card". Because if something is missing, you could be charged with theft, and anyone's reputation isn't worth $20... Not even $100,000.
But if the money was just left there... Then enjoy, buddy !