You may try:
select Distinct T0.CardCode, T0.DocNum, T0.DocTotal
from OINV T0
JOIN RCT2 T1 ON T1.DocEntry=T0.DocEntry AND T1.InvType='13'
JOIN ORCT T2 ON T2.DocNum=T1.DocNum
WHERE DateDiff(dd,T2.DocDate,GetDate()) <=30 AND T0.DocTotal != t1.SumApplied
Once you can get any results, change <=30 to =0