Worst customer service

I went to buy some groceries at Shoprite in THOKOZA khumalo street and i requested a cash back to be included on the same transaction but their cashier's who thinks the money in the till it's theirs claimed they didn't have cash the cash was on the money market and i was forced that if i want money i must buy airtime yet had already bought groceries and this is the 3rd i am experiencing this form of a treatment at the same shop, i have been in other shops i was never forced into buying something before in order to get money