Maybe this?

select NMDOS, case RowNum when 1 then ETOTALDEB else NULL end as ETOTALDEB, tpdesc, dataobra, NOME, OBRANO, nmdoc, FNO, etiliquido

from

(

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, ROW_NUMBER() over (Partition by bo.ETOTALDEB order by bo.DATAOBRA) as RowNum

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

) x

order by x.DATAOBRA

The problem here is that we are shooting in the dark. We can't see your tables, have no concept of your data or what you are trying to do. We can't test the code we post and we have no real idea if it will work.

If this doesn't do it then please take a few minutes and read the first article in my signature for best practices when posting questions.