A long, long time ago. You may remember when gasoline was under a buck a gallon? I was driving from southern Kansas to Iowa. I had everything I owned in my little car including a twenty dollar bill and some change. In the middle of the night I pulled into a 24 hour gas station, filled my tank, stepped up to the counter and handed the cashier my money, took my change, and drove until I found a place to park and get a few hours sleep.
I woke up, and drove onwards, until I had to stop for gas again, and I handed the clerk a twenty??!! I went back out to the car with my (correct) change.
The first clerk had handed me the change, and my original twenty back.
It was a mistake, but there was nothing I could do about it, if I drove all the way back I would never get home, plus I was very hungry. So I bought me a cheap breakfast and a glass of water and drove onwards again.
I still feel sorry for the clerk responsible but I didn’t steal it, and I still consider it worse than the OP’s little scenario. Fifteen dollars is a little over five gallons, it was already paid for, and there is a plausible explanation, in order for the pump to auto shut off, it had to be prepaid inside and registered to the pump, probably in cash. Who goes inside to prepay with a credit card when the pump is set up to do the transaction. Not very many I am sure.
Having been poor and living out of my car, I know the feeling of not looking a gift horse in the mouth. If I had found the mistake before I had left the parking lot, I would have been put to the test, and would feel much worse than I did if had just kept going. So for purplehorseshoe, Merry Xmas. Santa came early for you.