Never not been able to pay for fuel, does happen quite often at work though, not allowed to let people pop home anymore either due to the few who haven't come back, though that doesn't stop me if it's somebody I know.
So would stopping under a fiver or at least apologising
Don't know if it's just the way I'm wired, but I'll happily let everybody off the odd change (within reason, no mate, 78p isn't going to happen lol...) as you can guarantee at least a couple of people will leave you change during a shift to balance it all out. But people who make no effort or flat out refuse just look like arrogant ***** and will usually get loudly informed that if they lack the ability to control their pump hand then they should perhaps learn that they can underfill. Make the slightest effort to even appear to look for change or just say 'sorry mate, I don't have the odd' and you're golden