What if we were to take a discounts module, and add some more logic and functions to it. Then in a merging module, hide the discounts and have their settings set by the admin/store owner. At checkout time, take the "discount" (obviously not shown as a discount to the user), and create a new invoice at the invoice completion time for the amount of the discount. If you need multiple installments, add some logic so that at checkout of the next invoice, another invoice is created again with the remaining "discount".

Eh?