I issue a single purchase order for many products. They arrive in multiple shipments over weeks or months. For every arrival, there is a receive of products and the supplier issues an invoice. All the invoices put together will add up to the purchase order but each invoice must be paid on its own terms, not once all invoices have arrived.

For example. Purchase order for 100 widgets. 25 widgets arrive along with an invoice for 25 widgets. I can receive 25 widgets into stock. How should I enter that partial invoice and make a partial payment on the total due for the PO?

When viewing the PO, if you click <Receive Invoice>, you can edit the invoice form and enter 25 widgets and pay for that quantity. Then, if you go back to the PO and again click <Receive Invoice>, then you are taken to that invoice you just paid for 25 widgets. There is not, as hoped, a new invoice for the balance (90 units) of the PO.