Try:
SELECT T0.[DocNum], T0.[DocDate], T0.[CardCode], T0.[CardName], T1.[ItemCode], T1.[Dscription], sum(T0.[DocTotal]) FROM OINV T0 INNER JOIN INV1 T1 ON T0.DocEntry = T1.DocEntry WHERE T0.[DocDate] between [%0] and [%1]
group by
T0.[DocNum], T0.[DocDate], T0.[CardCode], T0.[CardName], T1.[ItemCode], T1.[Dscription]
union all
SELECT T0.[DocNum], T0.[DocDate], T0.[CardCode], T0.[CardName], T1.[ItemCode], T1.[Dscription], -sum(T0.[DocTotal]) FROM ORIN T0 INNER JOIN RIN1 T1 ON T0.DocEntry = T1.DocEntry WHERE T0.[DocDate] between [%0] and [%1]
group by
T0.[DocNum], T0.[DocDate], T0.[CardCode], T0.[CardName], T1.[ItemCode], T1.[Dscription]