This is what I think about at the pump. More and more I see gas pumps ask if you want a receipt BEFORE the gas is dispensed. This seems risky. If you decline the receipt and then dispense gas, the pump could cheat on the amount of gas dispensed with less risk, as a papered record of the purchase amount and price is not produced. If on the other hand, the pump waits to ask if you desire a receipt until after the gasol…
But according to this theory, it sounds as if one should ask for the receipt afterwards?
However, in the pumps he's talking about, the machine prompts you beforehand: "Click here for a receipt". The receipt decision (yes or no) is always before. There is no way to ask for a receipt afterwards.
He always asks for a receipt (even if he doesn't need the receipt) because he figures that the machine is less likely to cheat him in that case.