Turkish Airlines Ticket Cancellation Issue Due to Booking Mistake

I purchased a round-trip flight ticket from Adana to Istanbul with Turkish Airlines. However, I mistakenly booked the return ticket from Istanbul Airport instead of Sabiha Gokcen Airport. I realized this mistake immediately after booking and called customer service within 5 minutes. I explained that I would be returning to Sabiha Gokcen Airport from abroad on the same day and requested to cancel the ticket booked from Istanbul Airport. However, the customer service representative was not helpful at all and stated that I could only cancel the ticket with a deduction, and it would apply to both the outbound and return tickets. I mentioned that this situation would cause me significant inconvenience, but they informed me that a ₺800 deduction would be made for the ticket I called to cancel immediately after booking. I have never encountered such treatment with any other airline. I don't understand how customer satisfaction can be so disregarded. Since I noticed the mistake and called right after booking, I request a full refund without any deductions. Otherwise, I will be severely inconvenienced.