Why refusing to give a till slip?

<p>I regularly buy kiddies clothes at Ackermans. A while ago I bought a kids bodywarmer, but it turned out to be too small. Unfortunately I lost the receipt for that purchase, but when I bought from Ackermans again I learned you can still exchange purchases if the item is unused and tags is still attached. So I returned the item and a male cashie told me I could go look for something else while he makes a copy of my ID. There was not a larger size available, but I bought 7 other pieces of clothing which together was more than double the price of the returned item and I paid up the difference. The cashier lady handed me the store receipt I had to sign and then my receipt. As I turned to go, the lady who did the override on the transaction told me I should hand back the slip the cashier gave me. Meaning I have no proof of purchase. Upon querying it she reluctantly made me a copy. I never had this problem before and it's not stated on the terms and conditions. Is this standard practice?</p>