Refund Complaint

I was at Pep stores clothing in Three Rivers (Vereeniging) to return an item for cash back refund but was not assisted . I bought school uniform on the 29th December 2017 and on the 30 December 2017 I return 1 grey trouser for a smaller size and they took the original slip and just gave me a return slip which only had the one of trouser on it . So logically I figured I did not need to keep slip ,so now today 26 Jan 2018 I went back to return another gray trouser because its also big ,however ,I needed a refund so I explained to the cashier as well as well the manager that I need a refund but no longer have the original slip as they took it on the 30th Dec. So the manager demanded that I should bring the return slip which I was issued on the 30th which I no longer have as if I'm satisfied with a product and have used the product I don't keep the receipt . The trousers I was returning for refund still have the price tags ,so I don't understand why I can't get my money back because its not like I lost the original slip ,furthermore ,they have the slip and I even told them the date of purchase .