Is changing sales figures to avoid Sales Tax haram, even if it’s common practice?

I am working as an accountant. My job description includes preparation of financial statements for the company. Since I am also responsible for sales section, my owner wants me to amend the sales figures in order to avoid payment of Sales Tax. Now I am in double mind whether to change that or not. Since it is unethical and illegal as well as per government laws. Now I want to ask whether it will make my salary haram? If it does, so what should I do? Since it is common practice here.