What is the ruling on the work of an accountant who is forced to reduce the amounts of tax deducted from employees, after his repeated attempts to correct the matter, and is his salary considered lawful?

You have done well in clarifying the truth, enjoining good, forbidding evil, and a Muslim's fulfilling of his obligations in this regard will not be harmed by the misguidance of those who are astray. If you strongly suspect that the officials are manipulating matters to cause the loss of employees' rights, then you must inform them of this. Your work in the company is permissible as long as it is in what is lawful, and your salary is permissible. Your remaining in the work while advising the officials is better than leaving it, if it does not directly lead to what is forbidden or assist in it, because your staying might expose injustices and allow you to advise the officials. In offering advice, follow effective methods, with wisdom and good admonition, patience, choosing appropriate times, and utilizing da'wah (preaching) means, along with supplication and prayer for the officials' guidance.