--from this

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
 ORDER BY mutfactuur



--to this

SELECT MIN(MutDatum)AS MutDatum,MutFactuur
    ,
      MIN(MutBetalingskenmerk)AS MutBetalingskenmerk,
    Mut_Rel_ID
    , SUM(CASE WHEN MutPartNr=1 AND mutsoort = 2 THEN (mutbedragex+mutbtw) ELSE NULL END) AS topay
    , SUM(CASE WHEN MutPartNr=3 AND mutsoort IN (7, 3) THEN mutrekenbedrag ELSE 0 END) AS paid
FROM tblBH_Mutaties
WHERE Mut_Ab_ID = 71
GROUP BY MutFactuur, Mut_Rel_ID
HAVING (SUM(CASE WHEN MutPartNr=1 AND mutsoort = 2 THEN (mutbedragex+mutbtw) ELSE NULL END)<>SUM(CASE WHEN MutPartNr=3 AND mutsoort IN (7, 3) THEN mutrekenbedrag ELSE 0 END))