sales order quanity updation change

Dear,

i have a issue is that suppose i have asales order of quantity 10 and out of which 5 have delivered and invoiced and another 5 have been done OBD. but when i go to sales order change the quanity to 5 it showing me warning message and allowing to change to quanity in the sales order to 5 without deleting the deliveryof 5..How can i can restrict this with out delteing the exact quantity no body can change change the sales order quantity.

regards,

debesh