Is it permissible for a partner in a company to participate in a commercial project that supplies goods to that company at the same market price, knowing that he will not offer them any facilities that distinguish them from others, and are they sinful if they continue with him?

There is no objection to a partnership with an employee if his work is not related to purchasing goods, selecting them, or determining the selling party. However, if his work is connected to these matters, then it is not permissible to partner with him, because it is a cause for the company's rights to be diminished. If this is done with the knowledge of the company owner, then it is permissible; otherwise, everyone involved is sinful for cooperating in something forbidden.