What is the ruling of Islamic law on working ten hours a day while the employer announces it as seven hours to evade taxes, and is the money earned from this work unlawful?

If taxes are legitimate (permissible by Sharia), then it is not permissible to help the employer evade them, because Allah Almighty says: "And do not cooperate in sin and aggression." However, if taxes are forbidden (impermissible by Sharia), then it is permissible to circumvent them and to help the employer in doing so. This does not affect your salary if it is for permissible work that you complete as required.