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

This is usually caused by AR-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
    = ar.id) otherdate FROM ar WHERE transdate > (SELECT MIN(transdate) FROM acc_trans ac 
    WHERE ac.trans_id = ar.id);