Turkish Airlines Ticket Change Fee for Fully Refundable Ticket

I purchased a round-trip flight ticket between Turkey and America. I chose the non-refundable option for the outbound flight, and for the return flight, there were three options: non-refundable, partially refundable, and fully refundable. I selected the fully refundable option to be able to make changes to my ticket. However, when I called customer service, they requested an additional $160 for the change. When I explained the situation, they said they couldn't see the ticket details and couldn't redirect me to someone who could. How can they request money if they can't see the details? The tickets for the date I want to change to are much cheaper, and I wanted to postpone it by 3 months. I also have the conversation recordings to prove this. When I first bought the ticket, I called you, and you told me that with a fully refundable ticket, changes could be made without extra charges, which was the purpose of choosing this option. Additionally, I paid for extra travel insurance. I request a free change of my ticket. I am urgently awaiting your response.