Is it permissible to take the equivalent of two months' salary in exchange for extra effort and time I spent at work without compensation, knowing that the employer promised me this but did not fulfill his promise?

If there is no agreement with the employer on compensation for overtime, whether financially or through leave, and there is no professional custom in this regard, then the employee is not entitled to demand compensation, because in this case, he is volunteering his work. However, if there is an agreement or a custom, the employee is entitled to demand his right. If he is not given it, then there is a difference of opinion among scholars regarding the permissibility of taking it without the employer's knowledge.