I'm not sure, but which one of the payin_date's is your parameter?

see:

select dept_payin_no INTO DEPT_PAY from post_treasury_copy
where to_date(payin_date,'DD-MON-YY') = to_date(payin_date,'DD-MON-YY')

if you have a column with the same name, then it's obious why Oracle returns more than one row.