Is it permissible for an employee to take a monthly sum from workers he has brought to the company, without the company's knowledge, in exchange for providing them with work?

There is no harm if an employee endeavors for workers to find employment in his company or elsewhere, in exchange for an agreed-upon commission with them, even if it is a part of the worker's salary, provided that the duration of the commission is specified and the salary is known. This is permissible if his endeavor is outside his official working hours and is not part of his official duties, nor is he the one entrusted with selecting the workers.