I selected "Other" because it depends on the circumstances. In the grocery store, I just looked around and asked a couple of people if they had lost anything, and one guy felt in his pocket and said, "Oh, my Gosh! A $20!" so I gave it to him. (Probably equal to the $50 now - it was a long time ago.)
We found a small amount and there was a school group, so I went to one of the chaperons and asked her if any of the kids had lost some cash, and she hadn't heard anything, so I kept it and told her I had it. (It was like less than $3.00, the amount a kid would have after lunch and a small souvenir, maybe.) After the bus left, I just put it in the donation cash box. There was another incidence at a national park, and we put it in the donation box, too.
The $20 that blew into our back yard after a windstorm many years ago, we just kept.
I think you were very wise with what you did. You deserve it as much as the guy did, and if there had been a donation box and you didn't trust him anyway, it probably would have been a futile gift.
I don't know what I would do if it were a significant amount and would tempt me. I've never been tested that much!