Hi Scott:

One way of handling this would be to set up the product(s) to be invoiced based on delivered quantities and create a new UoM called "Percent Complete". You could then update each sales order line with the percent completion and generate the Invoice. Odoo will automatically generate the Invoice for the differential amount each time.

For example, let's say you have 2 products A and B. A has an order value of $100 and B has an order value of $200. You can book the order with an order quantity of 100 "Percent Complete". The unit price would be 1 for A and 2 for B so that the amounts of each line are the sales amounts.

Now, let's say A is complete 10% and B is complete 15% when the first Invoice is generated. You can update the delivered quantity on the order to 10 for A and 15 for B and generate the Invoice.

The next time, if the percent completion is 25% for A and 20% for B, you can update the delivered quantities on the SO to 25 for A and 20 for B. Odoo will then generate the Invoice for the differential amount i.e. 15% for A and 5% for B.

You will probably need to customize your report layouts if you do not want the quantity and unit price to appear on the SO/Invoice.

Hope this helps.