SELECT * FROM (
    SELECT
         MIN(MutDatum) AS MutDatum,
     MIN(MutBetalingskenmerk) AS MutBetalingskenmerk,
         MutFactuur,
     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
    ) AS m
WHERE m.paid <> m.topay
ORDER BY MutFactuur