Just curious about those saying it sounds like a scam...what's the play like?
The mere fact he saw her drop the money and did nothing is suspicious as hell and sets off warning bells.
There are several ways it could have played out. All of which depends upon the finder's greed. I'm sure they could produce the serial numbers or some kind of marking or other identifiers to prove the money was theirs. Your honesty stopped them in their tracks. I'll bet if you called the police they would know about them.