Unless there's some caveat that says 'if you got a discounted meal on the same receipt as a fully paid for meal then this specific receipt doesn't count towards

Unless there's some caveat that says 'if you got a discounted meal on the same receipt as a fully paid for meal then this specific receipt doesn't count towards the 8 receipts' then I don't really see how he's 'being difficult.' Don't advertise something you don't plan on honoring, and you don't have 'difficult customers.'