Here's another way. Just uncomment your code and comment out the sample data.

WITH CTE (NMDOS, ETOTALDEB, tpdesc, dataobra, NOME, OBRANO, nmdoc, FNO, etiliquido)

AS (

--SELECT distinct BO.NMDOS,bo.ETOTALDEB, bo.tpdesc ,bo.dataobra,BO.NOME ,BO.OBRANO,ft.nmdoc,FT.FNO,CASE

--WHEN FT.NDOC<> 1

--THEN 0

--ELSE FT.ETTILIQ

--END as etiliquido

--FROM BO left JOIN BI ON bi.bostamp=bo.bostamp left JOIN FI ON fi.bistamp=bi.bistamp

--left JOIN FT ON FT.FTSTAMP=FI.FTSTAMP WHERE BO.ndos='18' and bo.fechada=0

--GROUP BY BO.NMDOS,bo.DATAOBRA,BO.NOME,BO.OBRANO,BI.OBRANO,FT.FNO,FT.ETTILIQ,bo.tpdesc ,ft.nmdoc,bo.ETOTALDEB,ft.ndoc

SELECT 'Dossier 1',1000.00,10,20130210,'client',999,'Invoice',1.150,00

UNION ALL SELECT 'Dossier 1',1000.00,10,20130210,'client',999,'Invoice',4.250,00

UNION ALL SELECT 'Dossier 1',1000.00,10,20130210,'client',999,'Invoice',6.250,00

)

SELECT NMDOS, CASE rn WHEN 1 THEN ETOTALDEB ELSE NULL END

,tpdesc, dataobra, NOME, OBRANO, nmdoc, FNO, etiliquido

FROM (

SELECT *, rn=ROW_NUMBER() OVER (PARTITION BY NMDOS ORDER BY dataobra)

FROM CTE) a

ORDER BY DATAOBRA