copying control from billing doc to sales Order

Dear SAOians

I facing one problem. i have created a Billing document with milestone(TAO) and in the Invoice the Amount is different say 1000 and the net value is different 100. since this is a milestone and iam billing the first milestone. so system will show this difference. this is very much clear.

Now when iam creating the credit memeo Request with Reference to Billing document system copies the amount as 1000 and net value as 100. but iam creating the credit memo request for the first milestone. so it should take both amount and net value as 100.

is there any way to resolve this issue

Regards

Prakash