How to add the quantity for already billed item and do the billing for newly added quantity.

Hi All,

At preset we doing billing like below.

SO 123456

Item          Quantity

10               4       HRS

11                3      PC

Now we are doing billing total on 3 pc in based billing plan monthly or yearly.

Now requirement is like below.

the same above SO and item 11 now user will do the billing on 3pc this month 10.06.213 and after some time same time for same SO and same line item user want to add 2 more pc's and do the billling for remaining 2pc on 15.07.2013.

1) When I go to item level item 11 and 3pc which is already billed, so I am not able add quantity.

2) But i can add the quantity on schedule line item 2 PC's and it became item level 5PC's. But i need to do the billing for newly added 2pc quantity only. Likewise next time user will enter 4pc and need to billing different date with different quantity.

In my project there is no delivery related process, we have order creation, billing and invoice generation, because my project resource related,server license and server storage related.

Please let me know how to achieve this requirement.

Thanks.

Preddy.