wasn't mine

Today I encountered a rare event of having spotted a $10 note on the ground at safeway.
My first reaction was picking it up and gave it to the counter. The guy asked me "Do you want to keep it?". I said "nah" and left.

Now I couldn't quite understand why I did what I did. When ppl see money lying around without any obvious owner they think finders keepers, right? so why then? maybe $10 was too much and I don't feel comfortable taking it. Maybe I've never seen a $10 note lying around for grap. maybe I would've taken it if it was $2? What would the guy at the counter do? The money isn't in the system. Noone is gonna claim it. How can you declare ownership of lost money? For all we know, the guy prob kept it himself. maybe because I didn't need that money. maybe it didn't feel right. maybe I'm a dumbass.

Why am I thinking so much? Should've just taken the money and buy half a tube of toothmousse.