Brake Disc pads to be replaced 4 months later

I bought a car from them in January. According to the sales lady, the car underwent the Automark inspection. In May that same year the car went for a service. It was found that the brake discs needs to be replaced as there were only 3000 kms left on its clock. I contacted them and they agreed to look at it. I got so busy with the passing of family and could not take the car. I contacted the sales lady in July to ask whether they will still be able to look at the brakes for me. They agreed. After looking at the brakes they will not be replacing it because there is still 30 % left on the brakes. I mean when will they need to replace it...at 0%??? According to the sales lady they only cover me for 6 months after date of purchase. Which I personally think is not fair. I feel that the car was sold to me knowing that a few months later I would need to replace the discs. I should have been aware of this before the sale...not? I now have to pay R5500!