I never pay cash of gas... A few years ago my brother rented a RV out of Phoenix. He stopped for gas, paid the attendant with $100 bill and proceed on his way. A few miles down the road he was pulled over by the police and arrested for theft of service.
Apparently, the attendant pocketed the money and called the cops. My brother couldn't produce a receipt for the gas and it was his word (an out of towner) against a local. He had to post a $500 bond to get out jail and proceed on with the vacation. It was a tough break, but what else can you do in situation like that? He's from Vermont and where there's a lot of trust in your fellow man.