Weltbet Affiliates Refusing to pay commission

Weltbet Affiliates refuse to pay accumulated commission. They demand to deduct the listing fee they paid to add the reviews of their brand to our sites. They were aware that the Listing fee has nothing to do with the commission and it is a one-time payment for us to list their brand permanently on our portals. The unpaid commission accumulated since April and we did not receive any payment since we started working with them.