Is it permissible to work for the tax authority, and what is the ruling on a salary earned without performing work?

The default rule is that imposing taxes is impermissible due to the sanctity of people's wealth, as Allah Almighty says: "O you who have believed, do not consume one another's wealth unjustly but only [in lawful] trade by mutual consent," and the Prophet, peace and blessings be upon him, said: "The wealth of a Muslim person is not lawful except with his willing consent." However, the state may impose them in exceptional circumstances.

As for working in the tax authority, it is a condition for its permissibility that the employee does not work on unjust taxes. His salary is then lawful, even if there is no work, because a private employee is entitled to wages if he dedicates himself to the employer.