Purchase Order Milestone Forecasting

Need input on the following business scenario:

Business transacts with certain suppliers for long-lead, high-dollar amount Goods which are handled via Purchase Orders. These deals with the suppliers are agreed to be paid in the form of several milestones and the amount of the milestone is set at the time in which the deal is made to which each would be paid via its own separate invoice. The delay between each of these invoices however, could be months or even a year.

The problem which we are facing is that business needs a way to properly forecast and have visibility to these milestone amounts and estimated timeline of when these invoices will be paid.