Does an employee bear sin for financially participating with a company supplier, sometimes giving him priority, knowing that he supplies at a better price and quality, and none of his colleagues are aware of this? And what should he do if there is sin in that?

It is permissible for an employee to partner with a supplier outside official working hours, but it is not permissible for him to favor a supplier with whom he partners in supplying raw materials to the company without the company's knowledge that the employee is a partner of the supplier. If the company permits the employee to purchase from the supplier, then it is permissible; otherwise, it is not. This is because the employee acts as an agent for the company, and his purchase from a supplier with whom he is partnered is considered purchasing from himself. An agent's purchase from himself is not valid except with the permission of his principal.