I'd assume it was impossible as It would work like a supermarket till. When they print off the ticket the machine has registered the amount, say £1.50, so the bus driver should have gained £1.50. If he then takes money it will clearly flag up as a discrepancy. If he takes money and prints no tickets for you...maybe, but assuming other drivers do the same route it may look suspicious that his registered passenger numbers are lower then colleagues.
What should I do?