The factoring company (related or not, who knows at this point) will take the position that they paid New Century in full for the load and that you should do likewise, pay them in full. In normal circumstances, your recourse for the deductions would be to file suit against New Century for failure to provide the service you contracted with them. In this case however, New Century seems to have disappeared completely and legal proceedings against them would be a waste of time. The fact that you never signed the assignment letter is mute, as you have in the past paid the factor for New Century invoices, thereby establishing a pattern of payment in spite of the unsigned assignment letter. From the sound of things this situation will continue to get messy as more victims of this disappearing company come forward.