Overcharged and not refunded amount paid

I cancelled a hotel booking and was advised that a full refund would be processed. However, the refund received appears to be less than the amount originally charged, resulting in a disputed shortfall. I believe that the ***, described in the booking confirmation as an amount paid by Booking.com, was nevertheless included in the total amount deducted from their account. As a result, I contend that the refund should have been based on the full amount charged rather than the reduced amount paid. Several follow-up enquiries have been made to Booking.com without receiving a satisfactory response and expresses dissatisfaction with the lack of engagement on the matter.