Free gas

Last night I went to get gas at the corner BP. I handed my credit card to the cashier and she, without thought, pressed the cash button before swiping my card. I don't know how gas purchases work but you can't just go back and ring up some new gas or void the old purchase. so... they called their manager who tried to run them through some register procedure but nothing works. the cashier tells me "we'll bite the bullet on this one!" so i get free gas/// sweet. I only wish I would have put more in.... tee-hee.