SunExpress Flight Schedule Change Leads to Partial Refund and Additional Charges

I purchased round-trip tickets from Bremen, Germany, with a departure on 27/07/2024 and a return on 08/08/2024. Despite booking 30 days in advance, I paid ₺52,000 for the tickets. Due to operational issues, SunExpress Airlines sent a message notifying us of a change in our return flight time. When I called, they informed me that I could cancel without charge, and half of the amount would be refunded. Our phone conversations are recorded. Based on this, I canceled and rescheduled the tickets for another time and date, and they refunded ₺14,000 for the return flight, which is only half. They also deducted ₺12,000 as part of the so-called free cancellation. What does it mean to deduct money for a free cancellation? The current cost of a one-way return ticket is already ₺24,000. I plan to write to all relevant authorities and pursue my rights to the fullest.