What is the ruling on working for a Muslim who devalues profits in his books for tax evasion, and what is the ruling on the salary earned from this work?

There is no religious impediment to working with a tax evader, so long as the work itself is permissible. Evading unjust taxes is not forbidden, for they are like customs duties (mukūs), and the Prophet, peace and blessings be upon him, said: "The collector of customs duties (ṣāḥib maks) shall not enter Paradise."