Refuse to assist me

I bought something on Saturday using my debit card for my sister and it was on sale only to find out that it does not fit her then she took it back on Tuesday to adgars at maponya mall and she was told that they want my debit card before they can give her a refund ,what confuses me is to why should I give someone else my dr card for and what will happened in the case if I was staying in kzn or another province will I have to come personal in order to get my refund ? or I have to take it back where I bought those clothes even if Mamponya is the closest shop next to her , I asked for a manager I was told that she went out I must phone after 20 minutes ,when I phone again I was told that she was not on duty today, very sad and mind you my sister she is using money to go there and shes been there twice tomorrow will be her 3rd time