Is it permissible to market a product (that is not sold by the store I work for) in exchange for a commission from the owner of this product?

It is not permissible for an employee to engage in another job during working hours, except for the obligatory prayers and their supererogatory acts, eating, drinking, and what is customary, because that constitutes a breach of contract. Therefore, it is not permissible for an employee to promote another product not sold by the establishment where he works, unless with the employer's permission, or if the promotion takes place outside working hours and does not affect his primary job.