Urgent SQL Query

OPEN orabaan
declare
 @batch int,
 @ttype char(3),
 @doc int,
 @ttdr float,
 @ttcr float,
 @diff float,
 @s1r int,
 @s2r int

 
WHILE
 @@FETCH_STATUS = 0

BEGIN
 FETCH NEXT FROM orabaan

INTO  @batch,
 @ttype,
 @doc,
 @ttdr,
 @ttcr,
 @diff

if @diff < 0
update
 ttfgld102777
set
 t_amth_2 = (t_amth_2 + @diff)
where exists
 ( select top 1 t_amth_2 from orabaan
 where
  t_dbcr = 1
 ==> and t_btno = @batch
  and t_ttyp = @ttype
  and t_docn = @doc )
==============================

I want to add a clause to auto update t_btno when value occur as "t_btno" is variable int 

simple data

t_btno         t_ttyp                    t_docn           t_dbcr                 t_amth_2

17                 NI1                   20050205            1              -3914482498.0
17                 NI1                   20050205            1              -3914484153.3000002
17                 NI1                   20050205            2              -3914482497.3000002
17                 NI1                   20050205            1              -3914482497.3000002
17                 NI1                   20050205            2              -3914482498.0
17                 NI1                   20050205            2              -3914484153.3000002

I want to update first row only for each t_btno

thanks regards