What is the ruling on the wage received by an accountant who helps his clients conceal the true transaction figures to evade taxes?

If taxes are forbidden (haram), then there is no blame upon a person or an accountant for evading them or using stratagems to avoid them; because this falls under cooperating to repel injustice. The work of the accountant and his wages would be permissible if he works independently. However, if he is an employee, then he is not allowed to take wages for it. If taxes are permissible (mubah), then it is not allowed to use stratagems to avoid them. Concealing profits would be considered deception and cooperation in sin, and it is not permissible for an accountant or anyone else to do it or cooperate in it; as Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression."