How to book credit memo to reduce the Base amount and IL Tax amount

 Hi,
Please find the below custom requirement ,

Two scenarios are given below

For Ex:- We have raised RA bill on client worth say Rs. 1000/- and Service tax (India Localization Tax) for Rs 100/-

Case 1: He will pay the base amount Rs. 900 and the total tax amount of Rs. 100/-
Remaining 100 base amount he will not pay any more.

Case 2: He will pay the base amount Rs. 900 and the tax amount of Rs. 90/-
Remaining base amount of Rs.100/- and tax amount of Rs.10/- he will not pay any more.

Please confirm how to book credit memo to reduce the base amount and tax amount accordingly from the manual AR Invoice for the above two scenarios ?