Is the supervisor obligated to leave his job if the company, owned by a Muslim and producing jewelry, does not stop selling a small percentage of products bearing the cross symbol to non-Muslim customers, and is his salary lawful in this case?

The salary is permissible if there is no supervision over the manufacture of the cross. However, one must cease supervising forbidden things, as it constitutes cooperation in sin. The company owner must also be advised. It is not permissible to make a cross, whether for wages or otherwise, and its maker is cursed. Whoever receives compensation for a forbidden item should donate it as charity and repent from that forbidden work.