after some correction now it is working

your code

SELECT sales.billno,sales.billdt, sales.accode, acmast.acname, acmast.opamt, sales.qty, sales.amt, cast(0 as I) as idTemp ;
FROM sales ;
LEFT JOIN acmast ON sales.accode=acmast.accode INTO CURSOR tempcurs READWRITE 

update tempcurs set idTemp = recno()
select acname, min(idTemp) from tempCurs into filtCurs group by 1
update tempcurs set opamt = 0 where idTemp not in ;
  Select idTemp from FiltCurs 

Code after Correction

SELECT sales.billno,sales.billdt, sales.accode, acmast.acname, acmast.opamt, sales.qty, sales.amt, cast(0 as I) as idTemp ;
FROM sales ;
LEFT JOIN acmast ON sales.accode=acmast.accode INTO CURSOR tempcurs  ORDER BY 3 READWRITE 

update tempcurs set idTemp = recno()
select acname, min(idTemp) as idTemp from tempCurs into CURSOR filtCurs group by 1
update tempcurs set opamt = 0 where idTemp not in ;
  (Select idTemp from FiltCurs )
  
  SELECT tempcurs 
  BROWSE 

warm regards,
mk.