Is it permissible for an employee to buy and sell materials that are not part of the company's merchandise in which he works, in order to achieve personal profit, whether in the company's name or not, with the customer's knowledge of that and without the employer's knowledge?

If the worker is a private employee, he is not permitted to work for anyone other than his employer during working hours unless he has his permission. If he does so without permission, he is sinful. There is a difference of opinion regarding his entitlement to the full wage. If his additional work harms his work at the company, then all or part of what he earns from his other work, commensurate with the damage incurred by the company, may belong to the company owner. However, if he performs this work outside his official working hours, there is no objection, and he is entitled to what he earns, while being careful not to prioritize his own interest over the company's interest.