No show selection

HI I had a guest that stopped infront of our guest house and said that he needs accommodation, I told him that we are fully booked. (I had a private client call my directly to book in that night so it wasnt added on the calendar yet) then he still went and booked on booking.com, it didnt appear on our nightbridge calendar so I wasnt aware of it.

So he didnt make any payment or deposit and also didnt stay with us. I am new with booking.com so wasnt aware that I had to mark him as a no show (remember his booking also didnt show on our calendar) 

so now I need to pay commission for his stay that never happenend because I didnt mark him as a no show... I already marked the invoice as quiring commision but booking.com declined it.

Please help me =-(((