SELECT DISTINCT m.MutFactuur, m1.topay, m2.paid
FROM tblBH_Mutaties m
 CROSS apply (SELECT SUM(MutBedragEX+MutBTW) AS topay FROM tblBH_Mutaties WHERE 71 = Mut_Ab_ID AND MutFactuur=m.MutFactuur AND MutPartNr=1 AND mutsoort = 2) m1
 CROSS apply (SELECT SUM(MutRekenBedrag)     AS paid  FROM tblBH_Mutaties WHERE 71 = Mut_Ab_ID AND MutFactuur=m.MutFactuur AND MutPartNr=1 AND (mutsoort = 7 OR mutsoort = 3)) m2
WHERE m1.topay <> m2.paid
  AND Mut_Ab_ID = 71
ORDER BY mutfactuur