What to do if you see 10-20$ falling off someone pocket ?

I've actually had several occasions to give money back to people. The last one I can recall was at a grocery store. My step son found a roll of dimes on the floor. He wanted to keep it, but knowing how the cashiers start their tills with rolls of change (I used to work at this store as a cashier), I advised him to return it to the customer service desk. Someone would have been held accountable for that little bit of money missing from their register at the end of their shift. Lesson learned for my step son.

I think it's simple civility to return something that doesn't belong to you. I would be extremely happy if someone returned money to me that I had dropped, so I want to return the favors.

I have also had the fortune of finding cash just laying on the ground, no one around that it could've came from. In that instance, I'll look around, and if no one appears to be searching for anything, it goes in my pocket.