Declining a booking (wrong pricing)

Hello! 
We are very new to this with Booking.com and finding some of the processes/help topics tricky to understand. 
We’ve just had a booking for next year but the pricing wasn’t correct for some reason. I am sure we had gone through the dates carefully and altered the dates. 
We would lose £420 if that booking can’t be cancelled. 
We have declined the request via the message you get asking for a check in time, but the dates are still showing as booked.

Can we have some advice on how we sort this problem? 
thank you 👍