open quantity in credit memo and debit memo

Dear Friends,

I am facing one problem in billing cycle. I am creating credit memo directly with ref to invoice(With out cr memo request). I am describing scenario below.

I have created Invoice 8012003 with one item with 10 quantity, after that I am creating credit note with ref to Invoice no 8012003, here I am creating credit note with 3 qty. Sub subsequently when i create credit note with ref to invoice number 8012003, in open billing quantity system is showing 10 quantity instead of 7.

I have used copy requirements 027 and billing quantity field is disabled.

Kindly advice.

Kaushal