Hi,
As replied by Mr. Gordon approval query will work only on document level not on row level. You can create an alert for balance quantity for reserve invoice.
Try this query for an alert:
SELECT distinct (T0.[DocNum]) FROM OINV T0 INNER JOIN INV1 T1 ON T0.DocEntry = T1.DocEntry WHERE T1.[Quantity] <> T1.[DelivrdQty]
Thanks & Regards,
Nagarajan