Horrible service at Woolworths Springs Mall branch

I went to Springs mall to exchange a short pants which I had bought for my daughter. I had a slip and a price tag was still in the short the cashier asked me for the bank card that was used to make the payment. I told her that I don't understand why she needs a bank card firstly I have the slip and the price tag is still on the shorts also I am exchanging with another product why does she need a bank card after back and forth. She tells me because I bought those shorts 3 months ago she will only give me with the price on the system which is 100 less of which is not fair why do I pay a full price with slip and price tag then when I need to exchange I get exchanged with less amount.