Faulty tracktop that is negligence from their side but I’m paying for it

I purchased school uniform worth R9000 and including a tracksuit, the top of the tracksuit had a factory fault, I had marked the tracktop because I couldn’t have anticipated that it would be faulty as I believe they check their staff before packaging them, I returned it and they are saying they can’t take it back because I have marked it, it beats me how will my child wear a faulty item, the sad part is that they are the only supplier of Curro school wear, I’m so disappointed. I would understand if they refused to take it back because it’s SIZE(that would have been negligence on my side) but the FACTORY FAULT??? Item is not wearable.