Refuse to reverse wrong transaction.

My husband bought me a La Senza gift voucher of R1000 for my birthday. I proceeded to the Tygervalley branch to buy a specific item to wear with my birthday party dress. The item costed R700 it was not a sale item! At the till out of habit (and because of being in a hurry to my lunch time party) I handed them my debit card instead of the voucher card! Realizing my mistake I informed them just as the transaction went trough. The lady said sorry the transaction already went through, We can't reverse it. I repeatedly asked her if there was a way she could refund me, she kept on saying no. Me: Can't you reverse the transaction and take it from my gift card, my husband bought me the voucher as a gift and I cant spend double the amount at the store. L: \No