Rude manager, lay-by issues

I have lay_byd some items on the 14th of December 2020. I was called yesterday to say I need to go and collect my lay-by as it has expired. I then go to fetch the lay-by, I paid for the balance and opened the lay buy, when I open the lay-by I realise that I already have some of the clothing Items, so I request to exchange, when I exchange we realise that the items have now been placed on sale. The rude cashier then said to me I cannot exchange the items on the amount I bought them because I do not have a receipt I can only exchange them on the sale price, I said there is no way I will do that. I mean what kind of nonsense is that? You have the receipt to prove that I bought them at that price, surely you can check on the systems to see that the items were not on sale at the time of purchase/Lay-by. All I want is to exchange the clothes at the price I bought them that is all.