post_query trigger on a custom form

Hello all,
Thank you for taking your time to help me.

I am not getting an error, I am just getting nothing in my variable l_due.  If I run the sql individually, I get the expected return. 

  PROCEDURE post_query IS

     l_due number;
    
    BEGIN 

            -- CUSTOMER PAID DATE

        BEGIN
           
                l_due := 0;

            SELECT sum(DECODE (pay.invoice_currency_code, 'USD', pay.amount_due_remaining, (pay.amount_due_remaining * pay.exchange_rate)))   
          INTO l_due
          FROM ra_customer_trx_all rct
             , ra_cust_trx_types_all trx_type
             , ar_payment_schedules_all pay
         WHERE rct.ct_reference = to_char(:xxprk_brasil_v.so_number)