Vitra Artema Reservoir Set Warranty Issue

I purchased 3 Vitra concealed cistern sets. After installation, one of them started leaking water from the flush valve. I learned from the service that the faulty part is called a 'gasket'. They told me that both the service and the headquarters would replace it under warranty. The service technician visited my home and claimed that the product had scratches due to impurities in the water, demanding payment for repairs. I refused to accept that. I contacted the headquarters and they evaded responsibility, stating that since the installation was done by the service if the product got scratched, a fee would be charged. I paid 1900 TL for the complete cistern set. They asked for 1200 TL for a single internal part of a valve. We trusted Vitra for its quality, but if a tiny impurity can cause such damage, where is your quality now? Unbranded versions of the same product are available for 40 TL, with service, warranty, and durability. Just like low-quality companies, you're only interested until the sale is made. We used to consider Vitra a valuable brand in this country, but you've scored an own goal over a minor issue. Shame on you. I'd rather break the entire cistern set and install a cheap unbranded product than give you that 1200 TL. This is complete nonsense.