Playing this weekend, a couple of times we saw this scenario: Someone chose to keep more than one die, revealing them with everyone else, and then said, "Oops, I don't have the money to keep those dice." These were genuine mistakes on their part, not attempts to cheat, and we simply let them remove the extra dice. But is there some official way to deal with these situations? Or, is there a better way than how we handled it?
Pay attention. Seriously. Not much else you can do.
We sometimes put the dollars we are going to spend on top of the dice cup before we reveal so it is in your face. But I think this gives away too much info since you haven't revealed your dice already.
Or just be very regimented in how you reveal your dice and announce payment before moving on.