Booking.com Overcharged €526 for Mistaken Reservation with Only €21 Refunded

I made a reservation through Booking.com for a hotel in Greece. I intended to book from July 28, 2024, to August 1, 2024, but mistakenly selected July 18, 2024, to August 1, 2024. I canceled within an hour of booking, expecting a full refund due to the free cancellation policy. However, only €21 was refunded out of the total €526. I contacted Booking.com customer service, who acknowledged the mistake and assured me that they would discuss the refund with the hotel. Despite my follow-up with both the hotel and Booking.com, I was informed four days later that the hotel refused to process the refund. Both parties have been unhelpful and only offered apologies. This situation is unfair and unacceptable. I have filed a complaint with CİMER and will also file a complaint from Greece. This is a significant amount of money, and I am demanding a full refund.