Create a shipping cart price rule which gives a discount and the discount amount is the same as the TAX amount.
For example, if the total tax is $12 for the current customer's cart, then the shopping cart rule should add $12 discount to the cart.
The business logic is, for some products, assume tax is 8%, as per government rules we can not do anything with tax and we can not give any discount in the tax. So here what we want to do is we want to provide a discount to the customer and that discount amount is the same as their current cart total.