Disappointed

I bought an item from Ackerman (Sammy Marks Square in Pretoria) on Thursday 3 June 2021, then today the 7th when I try to wear that item I realized it still has that magnetic chip the put on items. The unfortunate part is that I can't find the slip. However the price tag is still there on the item and I still have the message on my phone of the purchase. However Ackerman refuses to remove the chip on the item if I don't have the slip, I have tried to explain to the manager but he didn't even want to listen to what I have to say, he just said " without the slip I can't help you". I mean is not my fault that the cashier didnt remove it, and also it didn't beep when I left the shop on the day of the purchase. So why am I as a customer have to suffer for their negligence.