Deducting 20% of purchase price when return goods

I had purchased a ladies pajama set from WW Chatsworth Centre for my mum for her birthday. The pants and top collectively came to approximately R370.00. I had requested a gift receipt from the cashier and was advised that there is no need for a gift receipt, if my mum doesn't like it she can simply return it at any WW store for something she likes or a gift card. My mum went to the Chatsworth Centre store yesterday to return the pajamas and take some casual wear clothing instead. She was advised by the customer services department that 20% of the purchase price will be deducted by WW as she does not have a receipt. My mum explained that I had requested a receipt and the cashier advised that there is no need for a gift receipt. After many attempts to explain my mum walked out of the store with the pajamas as they insisted on deducting 20% (R74) as there was no receipt. This is not acceptable what happened to \returning merchandise at the current pricing system". My mum had checked and the pajamas are not on sale/marked down etc."""