Workflow:

1. Create and confirm a purchase order - Vendor A:
   Product A, quantity 1, unit price $100
   Product B, quantity 1, unit price $200
   
   
2. When we pay the vendor $100 as down payment before we receive the vendor bill, we should create a vendor payment (from accounting application) for Vendor A

3. Received products with journal entries.

4. Create a real vendor bill

5. Before reconcile the $100 down payment on the full amount bill, Odoo shows the ‘add’ below the total amount.

6. Click on ‘Add’ to reconcile the $100 down payment with the actual vendor bill $300.

We should see the balance of $200 on the account payable.

7. Now the vendor bill balance (amount due) is updated to $200.

The vendor bill status is updated to ‘partial’.

See also https://www.odoo.com/forum/help-1/odoo-18-0-how-do-i-link-a-downpayment-to-a-purchase-order-265600