Is it permissible for an employee to take money from his company to invest it for his own private benefit and keep the profit for himself, on the pretext that the employer does not give him his full rights and makes him work extra hours without compensation?

The employer must pay the full agreed-upon wage, just as the worker must adhere to the specified work, as Allah Almighty says: "O you who have believed, fulfill [your] contracts." It is forbidden for the employer to reduce the worker's wage by exploiting their need, because that is a forbidden injustice. The worker has the right to demand their right, and if the employer does not respond, they may complain to the competent authority. If the authority does not give them justice, they may take the equivalent of their right from the employer's money, if they are safe from being accused of betrayal. This is based on the issue of al-Zafar (taking one's right) and the hadith of Hind bint Utbah. However, this worker's method of taking their right is not permissible, because it exposes the employer's money to loss, and they might take more than their right, thereby becoming an oppressor.