Valid everywhere except here

Every time we go to the 7 days restaurant (and I know it must sound like we go there a lot at the moment - I'm just trying to summon up the courage to cook something) we get at least three money-off vouchers. At least, we assume they're money-off vouchers, they have 20 RMB printed on them anyway, but everything else is in Chinese. It seems a bit unfair to use them when the bill is only 77RMB in the first place, but as we'd been for dinner tonight and the bill was 299RMB, it seemed fairly reasonable. We handed in one of them with the bill and 300RMB. A crowd of waitresses immediately gathered round in a sort of rugby scrum to discuss the situation. This included our regular waitress (Yompee I think her name is, probably not spelt like that) who knows our names, and who had actually given us most of the vouchers. They appeared to reach some sort of agreement, and Yompee cautiously approached our table. She most apologetically managed to tell us that we couldn't use the voucher, but she couldn't really explain why. We told her it was really no problem, and clearly relieved, she disappeared. She returned a moment later with 1RMB change and 7 more money-off vouchers.