"This is quite awkward, but I believe that is the dollar that I, in fact, just dropped."
"That is my dollar that you have just found. Perhaps I can give you a reward fee of 50 cents for finding and returning my dollar?"
"I am almost completely positive that the dollar in your hand belongs to me. You see, I just dropped a dollar in the same spot that you found that one. How do you suggest we relinquish this terribly awkward situation?"