Issue with £20 Xbox top-up/gift card

Hi, I recently purchased a £20 Xbox top-up/gift card and lost the code on the way home, I’ve contacted the retailer and they’ve said that since the top-ups/gift cards are not refundable they cannot provide me with a one time courtesy. They also said that
its impossible to reprint the code. I was wondering if there is any way that I can prove the code hasn’t been used to Xbox so that they or the retailer can send me a new code. I also have the master reiceipt i recieved from the retailer that shows the purcahse
of the gift card/top up with the date and time provided. Thank you

:)