Is it permissible for an accountant in a company to undertake the follow-up of accounts, invoices, and goods for one of the merchants in the same company for a monthly fee, knowing that this is not part of his primary job duties?

The employee is a private contractor, and their service is owned by the employer throughout working hours. Therefore, they are not permitted to work for anyone other than their company during working hours without its permission. They must perform their work for the entire specified time. Exceptions to this are the time for purification for the five daily prayers and the time for performing them with their emphasized Sunnahs, as well as Jumu'ah prayer and Eid prayers. It is not permissible for a private contractor to work for anyone other than their employer. If they do so and cause harm to the employer, the employer has the right to demand compensation for the lost service.