Trueworths Sale Refund denied

On the 22th July 2024 I went to Golden Walk Trueworths and bought my brother a body warmer, upon arrival at home I realised that it does not fit him he has gained weight a bit, I went back to the store on 4th August 2024 to ask for refund since they didn't have anymore sizes, they then told me I can exchange for some other item other than the one I wanted and unfortunately right now I don't want anything else other than the one I took.

The store is refusing to refund me, I do not have the till slip anymore but still have the bank transact proof of payment, which they refuse to allow for a refund. I provided the date, time and till that I did the transaction on when purchasing the item to trace on their side but they have refused and told me they only accept till proof of payment which I feel is a *******! The surveillance cameras can also help them to verify whether I was there on the 22 July 2024 at 13H08 at the till I showed them to trace the copy of the slip that remains at the store but they did not bother to check if that was true, and just took advantage that I lost my point of sale slip which is ridiculous!!