One thing I always taught my children about lying was “you don't want to lie because God can't bless a lie, and your hanging out there by yourself and you are no match for satan. It's a scary place!”
That same principle applies to not keeping money you know doesn't belong to you.... you place yourself in a scary place.
Last year I gave a 20 back to a Wal-Mart cashier who accidentally handed it to me instead of a 10. I’m an atheist.
It’s basic consideration for others. If you want a selfish motive, I don’t want it on my conscience that a minimum wage cashier came up $10 behind when turning in her drawer because I wanted to pocket $10. It’s basically theft. Regardless of your religion, society will become a pretty crappy place if everybody’s looking out for himself at the expense of others.