How to apply discount after tax amt.

Dear Team,

My requirement is user want to issue a free item but tax should be calculated on list price and invoice should be generated of only tax amt. Below is an example of calculation.

The scenario is suppose item A is free item. Item A price is 10Rs per unit.

Now, calculation is Item A : Rs 10
Tax assume : Rs 2
Total Value after tax is Rs 12 per unit
User requirement is price of A item should be deducted from total value i.e 12 -10 and invoice should be generated of only tax amt.i.e Rs 2.
Can you suggest how the steups can be done?