cash crusaders (power bank)

on the 17 of July I bought a power bank at cash crusader in Pretoria Tramshed and after two weeks I realized it not charging, it charge up to two bars. I contacted the store and told me to bring the power bank with the receipt or proof of payment. and when I get to the shop the store manager told me that the power bank I purchase is out of stock and refused to exchange for a different power bank because I didn't have a box.

I told them I needed a power bank or refund but the store manager refused.