An Appalling experience and undermining of customer

I bought a baby gate at Dischem President Hyper on the 05/06/2025. It wouldn’t fit between my wall. I went back on 06/06/2025 to purchase an extension of the gate, only to find they have it in stock. When I wanted to exchange it with the one that has an extension, they blatantly refused and told me the packaging is damaged. They refused to take it back or refund me as a result. They even called the manager who repeated what his employee had said to.

What I don’t understand: Why should it be a customer problem that the shop doesn’t have the extension of a product that it should have under normal circumstances?

I would really want to have my money back