I'd hand it in -to the shop nearest where I found it - yes the person make keep it, but that is their responsibilty and not on my conscience. I've had money handed into me as a shop assistant before, held onto the money for a time at the store and then I think we put it in the charity tin when no one claimed it, so I'd like to think there are other shop people who'd do the same.
I found a largish sum in a public hall once and handed it into the police. when no one claimed it, they gave it back to me.