It would be great, if user can do prepayments on sales invoice. Reason: At the creating of Sales Invoices from Project Management module, you can not use those functionalities like you have at sales orders. Here it is necessary to have them also in Sales Invoice.
Also, when a user now can proceed from a sales quote to sales invoice (user has now the change of opportunity) he wants also to adress his open sales invoice to the customer through a prepayment invoice.