By default, the direct debit system auto-debits the current invoice amount or payable amount, whichever is lesser. Here are two scenarios to give you an idea on how this works:
Scenario 1
Billpayer A has an outstanding balance of £150 and a new invoice #002 has been issued for £300, making his 'current invoice amount' £300 and 'payable amount' £450.
The amount auto-debited from his account on the due date will be £300.
Scenario 2
Bill payer A paid in an excess of £100. AA new invoice #002 has been issued for £300 making his 'current invoice amount' £300 and 'payable amount' £200.
The amount auto-debited from his account on the due date will be £200.