Assist in getting a bigger t-shirt size

On the 27th of November 2019 at about 12h00 i was shopping for my son and husband. As i was in the queue to pay we were told their cash machines are offline that was after 45mins waiting in the queue. I then left and return around 16h00. I shopped and paid the for the clothes. The line was long and only 2 tills were open. The clothes i bought for my son had the slip and went with the slip to Eastern Cape. The t-shirt i bought for my husband is small and i have to exchange it for a bigger size. Now they telling me that is impossible. I have tried 3 truworths stores but they refused. Please assist.