This is also possible from the Vendor Perspective.

By default, a Service Product has a Vendor Bill Control Policy (on the  Product -->  Purchase tab) of "On received quantities" which requires the User who signs off on the Service Milestone to update the Purchase Order line:

You can also set the Control Policy to "On ordered quantities":

• You can then just enter each Milestone Vendor Bill when you receive it.
• You don't need to update the Purchase Order Received Quantity.
• You should have another way to verify and document that the Service was completed.

We recommend the default, since there is a Chatter message below the Purchase Order showing WHO signed off on the Service Milestone and WHEN this was done:

Note: Here I have also created a Unit of Measure "Milestones" but you can also use the default UoM of "Units" if that will make sense to your Vendor(s).

I have also seen Customers create PO lines like this:

They would update (or not) the PO as discussed with the same options above, and during Billing for 40% complete do something like this on the Vendor Bill line:

This would allow you to pay your Vendor $600.