It might be a little bit late but here goes my 5 cents!
Based on your description, I believe this is a case of recurring invoincing/billing (and for those still wondering, it doesn’t exist out of the box up to version 7.11.10)
It requires programing skills to achieve it. It requires the use of logic-hooks. Basically you duplicate the initial invoice.
First you need to add custom fields under invoices to keep track the amount of payments and another to keep track of the steps the invoice is (by stept I mean 0%, 25%,50%,75%,100%).
Then using logichooks you can duplicate the previous invoice once the status changes on the latest invoice (Updated as paid for example)
Now. The AOS_Invoices module utilizes modules AOS_Line_Item_Group and AOS_Products_Quotes and need to be taken into consideration when duplicating invoices.
Check this post for ideas:
Thanks,
AlxGr