Certainly no way around the CASE but I might do it this way so as to only have to CASE it once.

WITH SampleData (accountNumber, MaturityDate, ResetFreq, Amount) AS

(

SELECT 123 , '20140331' , '1D' ,100.00

UNION ALL SELECT 123 , '20140331' , '1M' ,2400.00

UNION ALL SELECT 123 , '20140331' , '' ,8700.00

UNION ALL SELECT 123 , '20140331' , '1Y' ,-99.00

UNION ALL SELECT 123 , '20140331' , '1M' ,299.00

)

SELECT accountNumber, MaturityDate

,ResetFreq=MAX(CASE rn WHEN 1 THEN ResetFreq END)

,Amount=SUM(Amount)

FROM

(

SELECT *

,rn=ROW_NUMBER() OVER (PARTITION BY accountNumber, MaturityDate

ORDER BY CASE ResetFreq

WHEN '' THEN 1

WHEN '1D' THEN 2

WHEN '1M' THEN 3

WHEN '3M' THEN 4

WHEN '1Y' THEN 5

WHEN '3Y' THEN 6

WHEN '5Y' THEN 7

WHEN '10Y' THEN 8

END DESC)

FROM SampleData

) a

GROUP BY accountNumber, MaturityDate;