Sometimes the amount shown on the creditor control account in the balance sheet does not correspond to the amount shown in the 'AP--Reports--Outstanding' report. 

This is usually caused by AP-prepayment transactions. Here is a sql query that will help you to quickly identify any such transactions:

    SELECT invnumber, transdate, (SELECT MIN(transdate) FROM acc_trans ac WHERE ac.trans_id = ap.id)
    otherdate FROM ap WHERE transdate > (SELECT MIN(transdate) FROM acc_trans ac WHERE ac.trans_id = 
    ap.id);