Booking.com insists of commission payment for penalty waver

We have free cancellation up to maximal 5 days before arrival. So, I got a booking 5 days before arrival (which is 50 % penalty for cancellation). They wanted "hot water for shower". We don't have hot water so I told them if this is not ok they can cancel free of charge till 6 pm (4 days before arrival). I even sent them a text message through phone. At 5:55 pm they cancelled and requested penalty waiver. I confirmed free cancellation because this was still inside the time frame I promised them. This conversation is also saved under this booking. Three weeks later I received an additional invoice from booking.com about incorrect reporting. Several times I contacted Customer Service and Finance. The last message I received from booking.com were from May 11th. Customer Service wrote I should contact Finance, Finance said I should contact Customer Service. Just now I received a reminder to to payment. I did not charge these people anything, I even sent a screenshot with me recent credit card transactions. Since May 11th no reply from booking.com but instead a reminder for payment (which is not correct because I did not charge anything from this booking!). I always pay invoices very much on time, mostly even the same day I receive invoices. What can I do? Please help!!

Thank you.

ARNOLD