Booking.com Mystery Charge (250 EUR) After Cancellation & Refund

Hello, I'm reaching out to address an unexpected charge related to my recent booking and its subsequent cancellation. I booked the Rooftop Balcony Dream Margareten with confirmation number 4808676760 on March 27. Following property renovations, which were communicated to me through your system, I had to cancel my booking. The property managed the cancellation very late, and I received a refund of 921 EUR from Rental Services Wien Aunt's account. However, I was surprised to find an additional charge of 250 EUR on my account immediately after the refund. This unauthorized charge lacks an explanation from either the property management or Booking.com. As the initial refund was processed appropriately, I'm puzzled by this deduction. This incident raises concerns about credit card security and transaction transparency on your platform. I kindly request:

Explanation for the 250 EUR charge post-cancellation.
Immediate refund for the unauthorized charge.
Steps to prevent such issues in the future to safeguard customer financial information.

Thank you for your prompt attention to this matter. Looking forward to your swift response and resolution.