Hi Justice,
The below fms works fine in my test db.
SELECT sum(T0.[Quantity]) FROM RDR1 T0 INNER JOIN ORDR T1 ON T0.[DocEntry] = T1.[DocEntry] WHERE T1.[DocNum] = $[ORDR.docnum] and T1.docentry = (select ordr.docentry from ordr where ordr.docnum = $[ordr.docnum])
Refresh regularly on document number.
Hope this helps.
Thanks,
Joseph