John Dory's voucher *******

I purchased a R500 voucher for my mother's birthday. Note; it did cost a full R500. First try, John Dory's (Baywest, PE) system crashed and my mom had to pay cash. It was her birthday. A little bit embarrassing. 2nd try, she was told she is not allowed to benefit from the special she chose. My argument, R500 cash was spent to purchase this voucher. What is the difference in terms of having to pay this amount or showing proof of payment in the form of a voucher? If it is a gift you wish to send your parents, spare yourself the effort. Rather send them an EFT into their account. I do not understand the principle of losing out on a special deal when you present a voucher that has been paid for in the full amount? An explanation would be great.