Exchange of goods

I bought a nice dress and a top from Edgars Hemingway’s for my Sister, I brought it home only to find out it’s small. I looked for my slip but couldn’t find it so I went back after an hour latest to the same till but was told the lady is on lunch... I asked the lady and she came back saying the manager says the system has changed and they can’t assist me even the same cashier I bought from earlier can’t do anything. Can I please be assisted as I’m stuck with new clothes that can’t neither fit me or my sister. I swiped my card therefore I can get bank printout for the transaction.