Hi,

The general methods below can be used in Odoo to handle backorders for incomplete commodities in purchase orders:

1.Backorders must first be enabled in your Odoo configuration, so be sure you check this first. Make sure the "Allow backorders" option is selected by going to Inventory > Configuration > Settings.

2.Make a buy order: Make a purchase order for the items you wish to order. Enter the necessary details, such as the vendor, the products, the amounts, etc., by selecting Purchase > Purchase Orders > Create.

3.Receive Partial products: Navigate to Inventory > Operations > Incoming Shipments when you only receive a portion of the ordered products from the vendor. Find and open the appropriate package for your purchase order.
Update Quantities: For the items that were only partially delivered, update the quantities received in the incoming shipment. Change the quantity in the product lines by selecting "Edit" from the menu.

4.Validate the Incoming cargo: Once the amounts have been updated, click "Validate" to verify the incoming cargo.

5.Check for backorders: Following the validation of the inbound shipment, Odoo will look for any remaining quantities that were not received. Backorders will be generated automatically if there are any.

6.View Backorders: Go to Inventory > Operations > Backorders to see the backorders. You can see the list of backorders as well as the specifics of the items and awaiting quantities here.

7.Backorders can be filled by creating a new incoming shipment for the purchase order and updating the amounts for the pending products once you have received the vendor's remaining inventory. Once this fresh shipment has been verified, the backorders will be marked as fulfilled.
You can also refer to the video mentioned below.Odoo Backorder

Hope it helps