Hi,
Try this:
SELECT T0.[DocNum], T0.[CardCode], T2.[DocNum], T1.[InstId], T2.[DocDueDate], T2.[DocDate], datediff(dd,T0.[DocDate] , T2.[DocDate]) as overdue, T1.[PaidSum] FROM ORCT T0 INNER JOIN RCT2 T1 ON T0.DocEntry = T1.DocNum left join OINV T2 on T2.[DocNum] = T1.[baseAbs]