where is the error? if exists

what a hell am i doing wrong? why i'm gettin this error?

let me try to explain:

the table [_AUX] receive a value from a asp page (word 'TOJ' was subimited by a form).

based on this information, i'm trying to create a trigger to update another table called [dbo].[TODOS]. but i have to select only one row randomly that has the 'false' value and then change it to 'true'.

check my code:

 IF EXISTS (
select top 1 A.flag
from [DBO].[_AUX] AS A 
WHERE a.flag = 'TOJ' order by a.ID_AUX desc
           ) 

 update DBO.TODOS set DBO.TODOS.[TOJ_FLAG]='true' 
 where  DBO.TODOS.[TOJ_FLAG] = 
           (
  select top 1 t.[toj_flag], t.TODOS
  FROM dbo.todos as t
  where t.toj_flag = 'false'
  ORDER BY NEWID()
           ) 

im receiving a error from sql management studio:

Msg 116, Level 16, State 1, Line 14

Only one expression can be specified in the select list when the subquery is not introduced with EXISTS.

plz help!!!!