Is it permissible for a company to demand its dues from a sales representative and use blank checks signed by him against him in the event of a customer's non-payment and the company's negligence, knowing that the representative has completed his procedures?

If the representative is like an agent or an employee, then he is not liable unless there is negligence or dereliction in safekeeping; because his hand is a hand of trust unless he transgresses. And if his signing of a blank check is a commercial custom, then this does not obligate him with liability, unless he violates the customary instructions and conditions; because custom is اعتبار (considered/given weight). So long as the representative has carried out the employer's instructions, he is not liable.