Ackermans layby

I did a Layby in April, so I had found that my mother had already bought clothes for the kids, I went back to Ackermans in mall of the south and asked for refund, so they asked for the slip and I can't seem to find it, they insisted that they want to give me a voucher and I don't want it because a voucher they Wil be forcing me to buy from them. I just wanted my money back. I don't know whats difficult because I'm appearing on their system and the layby is also showing on the system. Why can't they just refund me back.