Hi, 

Try this code:Generate an empty Vendor Bill, and choose the associated Purchase Order (PO). This action will include all the lines that Odoo assumes require payment, taking into account items received across all inbound deliveries.

If necessary, eliminate lines not present on the initial bill (or adjust quantities downward). Odoo will maintain a record of these changes, and the next time you add the same PO to a new bill, it will include the remaining items.

Hope it helps

• Upload the Vendor Bill.
• Register the Vendor
• Use the field Auto Complete to look up.  This import the lines from the Purchase order including the QTY received.
• If all have been received before you register the first invoice, adjust the qty invoiced.  Then when you register the second bill, you will get the qty not billed. 

Depencies and recommendation:  On the product Purchasing set control policy = On received qty.