I lost my wallet one night when i was 19. I was very drunk and on someone's front lawn doing something very wrong, but also very right.
And the person whose lawn it was, found my wallet, along with evidence of my "wrong"doings, but they still tracked me down through the phone book. And told me that they'd found my wallet and returned it to me, with everything still in it, despite what I'd been doing the night before.
Which is why I would absolutely return the wallet with all of the money and everything still in it. If that person offered me a reward, I'd take it. But i wouldnt help myself to one.