SELECT     TOP (100) PERCENT YEAR(dbo.gbkmut.datum) AS 'Year', MONTH(dbo.gbkmut.datum) AS 'Month', dbo.gbkmut.reknr, SUM(dbo.gbkmut.bdr_hfl) AS Expr1, 
                      dbo.grtbk.bal_vw
FROM         dbo.gbkmut LEFT OUTER JOIN
                      dbo.grtbk ON dbo.gbkmut.reknr = dbo.grtbk.reknr
WHERE     (dbo.gbkmut.transtype <> 'V')
GROUP BY YEAR(dbo.gbkmut.datum), MONTH(dbo.gbkmut.datum), dbo.gbkmut.reknr, dbo.grtbk.bal_vw
ORDER BY 'Year', 'Month'

Year      Month     reknr     Expr1     bal_vw
2009        7        13004    64174.34    B

select year(dateadd(month, datediff(month, 0, t.datum), 0))    as [year]
       ,month(dateadd(month, datediff(month, 0, t.datum), 0))  as [month]
       ,t.reknr
       ,k.bal_vw
       ,min(ca.expr1) as expr1
from   dbo.gbkmut t
LEFT OUTER JOIN dbo.grtbk k ON t.reknr = k.reknr
cross apply (
       select sum(t1.bdr_hfl) as expr1
       from   dbo.gbkmut t1
       LEFT OUTER JOIN dbo.grtbk k1 ON t1.reknr = k1.reknr
       where  t1.reknr = t.reknr
       and    isNull(k1.bal_vw, '_') = isNull(k.bal_vw, '_')
       and    datediff(month, t1.datum, dateadd(month, datediff(month, 0, t.datum), 0)) >= 0
       ) ca
group by dateadd(month, datediff(month, 0, t.datum), 0)
       ,t.reknr
       ,k.bal_vw