How to link the AR Manual Credit Memo to Invoices

Hi,

We have an business requirement to link with the Manual Credit Memo's to Sales Invoices. The requirement details are mentioned below. Please give your valuable suggestions for this requirement.

1. Sales Invoice accounted to a customer for Rs.50000 and Invoice price is Rs.50

2. We are providing price discount for this Invoice on last date of the month.

3. Every Customers having more then 50 to 100 Invoices for the month.

4. We are calculate the price Discount manually for invoice wise and create one manual Credit Memo to the customer for Total value of the Discount amount.

5. We need to link all the sales Invoices to this Credit Memo.