Furious

I have been buying goods from takealot and it has been quite good, except this month I bought a product and the incorrect product was given to me. So I phoned the customer service and they told me it must be a return and I told them it isn't a return as it was the incorrect item. Now all of a sudden they tell me I have to return the product that they made a mistake with before I get my product. I don't see why I should be doing this when I have already paid for the item that they have not delivered. When I return the incorrect item, they should be giving me the correct item at the same time. Why should I now wait for my item that has already been paid for?