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:

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.