Can’t deliver on promises

I purchased a item online from Leroy Merlin only to be told after payment that they did not have the stock and can replace it with a item half the price. I was advised that they would refund me the difference in price. I have received the cheaper item but have not been refunded the price difference. I called Alice (from the Fourways Branch) to find out why I have not received the refund and I am still waiting for the response.