In the scenario an invoice is disputed on a retainer invoice, and a credit note is applied to the invoice in question, it would also be good to reduce the amount drawn on the retainer with the credited amount (a second option should appear after the credit note is applied to an invoice that is linked to a retainer invoice - "The invoice you credited is applied to a retainer invoice - do you wish to reduce the amount drawn of the retainer with this amount (increasing the balance on the retainer invoice to be drawn later) or do you wish to make a refund?)