I am setting up OpenERP for use with a travel services organisation. Payment terms for the services provided are related to the start date of the service being purchased and not the date the product was purchased and the invoice was raised. How can this be set up?

It is just the accountancy module we are looking at, with the products on an external system. When a service is purchased, we would like to raise a sales invoice for the full amount, with an immediate deposit paid against the account. Then a percentage of the balance is needed by the 1st January in the year of the trip. Then a percentage 10 weeks before the trip starts. Then the balance 2 weeks before the trip starts.

Trips will be purchased throughout the year, and so the sales invoices will be sent out at at any date. However, the payment terms do not relate to that invoice date. Is it possible (by a method I can't see, or by an extension) to enter payment terms specific to that invoice, with fixed dates for that invoice, rather than selecting from a global list of generic payment terms?