CREATE TABLE TT
 (YM VARCHAR(7), DD VARCHAR(2))
GO

INSERT INTO TT
 SELECT '2007-02', '30' UNION ALL
 SELECT '2007-03', '3'
GO
 
 
SELECT * FROM TT
 
SELECT
 YM
 ,DD
 , YMD = CASE WHEN ISDATE(YM + '-' + RIGHT('0' + DD, 2)) = 1 THEN YM + '-' + RIGHT('0' + DD, 2)
    ELSE
     CONVERT(CHAR(10), DATEADD(DD, -1, DATEADD(MM, 1, YM + '-01')), 120)
   END
FROM
 TT