Partial Shipment on Drop Ship Orders / POs

We need the ability to convert a Drop Ship PO to Advanced due to vendors shipping partial frequently

For example we currently have a $10K US order that has delivered except for about $500 of it that is out to october

We need to bill for the $9500.00 and really do not have a way to do so and keep the accounting right. 

Our vendor requires we pay them NOW.