Voucher ****!!!

This store is a ****. So they gave me a voucher of R50, i went to use it but the size was small when i went for exchange they tell me voucher is no longer valid i must pay the difference. They can't even show you those terms and conditions. Its funny cuz even the staff in the shop didn't know about this I'm sure they would've told me. so where in the paper its written that it's a once off???? I want my item and I'm not paying anything extra.