Bad customer service

I have a really bad experience at naartjie at East Gate mall I was doing an exchange for a smaller size with a gift receipt the cashier told me that that the item was bought on an R100 discount then I clearly mentioned that if its not possible for me to do an exchange I would like to go back with my item she the proceeded and said she'll check for me if its possible for me to exchange only to find out she did proceed doing a refund and gave me a credit note of R300 and said I must add R100 in order for me to exchange which was ridiculous to me cause I did not ask for any of that she had so much attitude while she was the one that did a process I did not ask for she must really fix her attitude cause what makes it worse she proceed and helped other customers without attending to my issue now am at a loss.