SELECT * FROM
(SELECT
    x.MutDatum,
    x.MutFactuur,
    x.MutBetalingskenmerk,
    x.Mut_Rel_ID,
    x.sBedrag,
    (CASE WHEN x.sBetaald IS NULL THEN 0 ELSE x.sBetaald END) AS sBetaald2
FROM
(SELECT
    MutDatum,MutFactuur,MutBetalingskenmerk,Mut_Rel_ID,SUM(MutBedragEX+MutBTW) AS sBedrag,
    (SELECT TOP 1 RelCode FROM tblBH_Relaties WHERE Rel_ID=Mut_Rel_ID) AS RelCode,
    (SELECT SUM(b.MutRekenBedrag)
     FROM tblBH_Mutaties b
     WHERE b.Mut_Ab_ID=71 AND b.MutPartNr=3 AND (b.MutSoort=7 OR b.MutSoort=3)
      AND b.MutFactuur=tblBH_Mutaties.MutFactuur AND b.Mut_Rel_ID=tblBH_Mutaties.Mut_Rel_ID) AS sBetaald

 FROM tblBH_Mutaties
 WHERE Mut_Ab_ID=71 AND MutPartNr=1 AND MutSoort=2
 GROUP BY MutDatum,MutFactuur,MutBetalingskenmerk,Mut_Rel_ID) AS X) AS Z  WHERE z.sBetaald2<>z.sBedrag