Refunded HALF of my money

Ipurchased my 75 year old grandmother an electric blanket to help with her arthritis and swollen joints and it stopped working before it reached its warranty- so we should get compensated.

I logged an EXCHANGE for another blanket, but instead you REFUNDED me HALF of the blanket's money. The blanket is valued at R899 but I was credited R479. The person from your call center said that I should have logged an exchange then it would have been replaced- I told him I did this, and he apologised for the mix up and said "but unfortunately you've already been credited HALF of the money otherwise we could have had it replaced."

So what now? It's not like there wasn't any stock available, I saw this blanket on your site and it's still there. Why must this be my fault?