It would be different if you found sixteen dollars just laying on the floor or in the parking lot. Then I would say it would be yours to keep, but in this case you know that the money belongs to the gas station and it was wrong for you to take it. I once saw some money fall out of a girl's pocket. I picked it up, but it would have been wrong for me to keep it because I had seen where it had come from. I returned it to her.
"Evolve thyself and lose all hate...." Orphaned Land