Although, in London, no one pays in cash, nor are tickets issued (unless the bus terminates early and you ask for a transfer voucher) - are there any other circumstances?
Interestingly, I had just this happen to me last week. I was very surprised when touching on (Oyster) on the bus to see a ticket issued. It was a notification that my Oyster Card had gone into a minus value and would only be accepted for that journey. Normally using the tube, you don't get that sort of message at the gate line.
"There was no valid pass or enough money on your card, so an emergency Oyster fare was charged. You must top-up the card to use it again, or you could use a Contactless Payment Card in future. THIS TICKET IS ONLY VALID ON THE BUS ON WHICH IT WAS ISSUED."
Which presumably means if you drop into "minus value", you cannot take advantage of the hopper fare.