salam. can i work as a tax consultant for people who have haram/mixed income? if so, shall i reduce their tax if the tax is of prohibited type?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger. {C}

We have already stated in several Fataawa that it is not permissible for the government to impose taxes on people's money unless it wants to provide with such taxes essential services, such as paving roads, building hospitals and schools, and other public services. However, the condition is that the government should have no money in the treasury of the state to cover these objectives, and provided that none of the money (taken from the people) will be spent on corruption or on matters other than the public interest.

If these conditions are not met, then it is not permissible for the government to take taxes, because in such a case, it is considered as Muks (taking money unjustly and without right), and this is one of the grave major sins.

Therefore, if the taxes that you collect are forbidden taxes, then it is not permissible for you to collect them, because doing so is helping in sin. Allah Says (what means): {And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty.} [Quran 5:2]

There is a stern threat about taking Muks from people. ‘Uqbah ibn ‘Aamir narrated that the Prophet said: “The one who takes Muks will not enter Paradise.” [Abu Daawood, and Shu’ayb Al-Arna’oot classified it as Hassan]

The fact that some people have mixed money that includes lawful and ill-gotten money, does not justify this injustice. The government is obliged to prevent people from forbidden earnings and not impose Muks on them under the pretext that their money is mixed between lawful and forbidden.

For more benefit, please refer to Fataawa 316597, 203644, 89947, 85356 and 129786.

Allah knows best.