The till operator has the functions within the till to take payments on account sales should the business allow it and the operators till security let them perform the task. A button can also be added to your touch screen design.
When logged into the till:
Administration -> Add Invoice Payment
Choose the debtor.
Select the correct invoice to make the payment against from the list and select "OK" (in the example above there is only one outstanding invoice in the list).
You can now click on to the normal payment screen the operators use for all regular sales and take payment normally.