Turkish Airlines Charged Me an Additional Fee Even If I Had The Right To a Full Refund

I received an email from Turkish Airlines (THY) stating that I had the right to a full refund and free changes due to a modification in my flight. Based on this email, I made a change to my ticket through the app, but the site charged me an additional ₺1128.02. To avoid any issues at that moment, I made the payment and then wrote to the THY feedback center with the necessary information and screenshots. The response I received implied that they had not granted this right and that the additional charge was my fault, stating they couldn't refund me because I had already made the payment. We trust the company name and purchase flights, but we are being victimized by their mistakes (changes and additional charges despite being told it was free). I will file complaints with all relevant authorities, especially the Consumer Arbitration Committee, regarding this issue.