Early Settlemnt Discount

 It is common practice in the UK (and other countries) for larger clients to take a settlement discount when paying invoices, for example discounting the invoice value by 3% when paying within agreed terms eg 30 day terms. 

There is no facility on Dear to alloow for this within the customer terms and so the only workaround I can see is to undo the invoice and make the adjustment 'after the event' which could create problems especially if the process reverts back to estimate or order.

Are there any other ways to handle this (the full invoice value must be submitted as the customer takes the discount on the invoice full value)