Give me my slip back & **** service

Went to Pick n Pay tonight to buy essential items, bought sweets on special, buy 2 for a discounted price. I purchased it and used my smart shopper card, it showed the discounted amount on the monitor but never deducted when I paid for it. I came back to the cashier and she and the manager had an appalling attitude. They deducted my total and refunded me. When I asked the cashier for my till slip as I needed to keep the slip for my other items purchased, the “supervisor” interrupted and said no I cannot get it as it is their proof. When I wanted to purchase another 2 items for the same price, the “supervisor” reared up the price and told us that we will have to purchase it for the full price.