What is the ruling on working in a government department whose land was usurped?

It is not permissible for a Muslim to work in a usurped place, even if the usurping entity is the government. This is because it is considered aiding the usurpers in their transgression and an affirmation of their falsehood, falling under the category of cooperation in sin and aggression, which is forbidden in His (the Almighty’s) saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." The righteous predecessors kept away from oppressors and their assistants, as Sa'id ibn al-Musayyab said: "Do not fill your eyes with the assistants of oppressors except with disapproval in your hearts, lest your righteous deeds be nullified."