Error on [URL Removed] which I must now pay for

I ordered a Huawei E5577 router from [URL Removed] but when it arrived they sent the wrong router. By then the E5577 was out of stock. I said I was happy to wait until it came into stock but they insisted on passing a credit and then using it to get the item when it came back into stock.<br> <br> It was back in stock within a week or two but now the price has gone up by almost R400 (from R1498 to R1879) for the same item. Now they say that due to their error (not mine) I must now fund the difference which I find unacceptable as it was not my mistake.<br> <br> I had subsequently ordered the item from another supplier but they were out of stock although they said they would provide it at the same price when it was back in stock but I opted for a refund as I had the [URL Removed] credit.<br> <br> I just want the router in exchange for the credit I have as originally contracted as it was a mistake on [URL Removed] part not mine which I am not willing to pay.<br> <br> I have contacted them but they are adamant I must pay the difference.