If someone drops a $50 bill on the street and I come along after they are gone and pick it up, it is mine. It is not stolen. It is lost. I am not liable for the other person losing their $50.
What? Really? You'd do that? You'd not be like: "hey, you lost 50 bucks".
Man, look up Karma, you poor soul.
Let me throw a hypothetical at you. Suppose I DO see someone drop $50 but instead of either telling him about it or picking it up for myself I choose instead to ignore it and someone else behind me picks it up and keeps it. Have I committed a crime then?