I would try my best to give it back to the person it belonged to.  I would  turn it in to the police and tell them all the information they needed to get the wad of  $100 back to the person it belonged to because that is a decent amount of money.  I’m convinced that they are wondering were their $100 bill is.  What if they went shopping for something they’ve been saving for years?  What if it meant a whole lot to them?  Would you rather be honest and give them their money back or keep it.  if you choose to keep it you broke one of Gods ten commandments.  If it was you who lost it how would you feel if somebody kept it to themselves.  If God wouldn’t do it please don’t do it.