Is it permissible for an employee who works for an unjust, impious sponsor (kafeel) who circumvents laws, to take payment for overtime work that he performs in place of other employees who have left, especially since the sponsor refuses to pay for it and does not respond to requests to provide a replacement?

What the company owner does in terms of injustice and deceit is forbidden, and it falls under the supplication of the Prophet, peace and blessings be upon him: "O Allah, whoever is given any authority over the affairs of my Ummah and makes things difficult for them, then make things difficult for him." And his saying: "Whoever deceives us is not of us." However, it is not permissible for the worker to take extra money without an agreement, because that is consuming people's wealth unlawfully. He must refuse work outside the contract, or resign, or be patient.