Use cases:

Costing Method: Average Cost

Sale order of Sofa for 5 quantities. Total Order value= 5000, per quantity price = 1000, Cost price = 200

Invoice policy: Delivered Quantity.

Customer pay full amount at the time of order as a down payment. 

Now in 2 days we deliver 3 quantities out of 5 and I generate the Regular invoice to recognize my revenue and COGS for 3 quantities only.

When I generate the regular invoice system not allow me to settle partially down payment and generate the credit not for the same.

Regular Invoice amount: 3000
Down payment amount: 5000 (Here we want to adjust only 3000 and system not allow us if we change in the next invoice we are not able to get rest 2000)
Amount due: 2000 (Due to this system not allow me to confirm regular invoice and convert regular invoice into credit note)

In this case how can I control my COGS and revenue account based on the delivery?