poor customer service

I bought my child short pants at Eastgate mall around Dec 2015. The pants are too tied and wanted to return them. I stay at kempton park and had bought at Eastgate because it's big and has variety. I don't usually shop there as the mall is far from me. Now yesterday 6 Jan 16 I wanted to exchange the pants but I have lost the slip. If they had those pants I would just changed the size. No Mumsy at Festival mall tells me I must know the date I bought those pants so that she can trace the receipt. I have exchange previously at edgars without a slip and it was not a hussle. Why it's as if someone is putting this rules to make it impossible to exchange. The price tag was still there which for me it's evident enough that the item was bought at edgars. I didn't want cash just to swap. Now I have to keep those pants because I forgot the date I bought it. Can u confirm if this is how edgars does business so that I can peacefully forget edgars and never buy from the shop. Many shops do exchange without slips. I did not intentionally loose the slip I know I had to have it but if u care enough for your clients you will have a way to accommodate such.