Return/Repair/Credit?

I feel very bad in what has happen. I bought my daugter a Proline Tablet on one of takealot's daily deals in December. Paid for it, then they did not have stock. This was then sorted eventually and we were given a different model, we were happy. Now the tablet has malfunctioned, screen has gone black. This product has always had its rubber protective cover on, it had no cracks on screen when I took it to be exchanged. First they said, that the item cannot be exchanged but they have credited my account with the amount I paid for the item. I rep**** that I do not want the credit as I would not be able to buy another tablet with the credit as the tablet I bought was on special and was really a good deal. i asked them to replace it or fix it. They then rep**** to say they cannot fix it as there was a crack. I told them there was no crack when i took it to them, they said it was inside and due to policy they cannot have it fixed. But why give me the credit I thought, somehow they think that it will cost them less to credit my account than replace the item due to it costing more than credit. I then pushed for them to fix it and then they rep**** that the credit was an error. Answers?