1. Receive the used car:

• DEBIT to INVENTORY for $15k (increase value of Inventory)
• CREDIT to INTERIM RECEIVED for $15k (hold the counterpart for the increase until we have an Invoice for it)

Note: you may have to unhide the Unit Price field on the Receipt screen, so you can specify the price of each car you receive into Inventory, otherwise you would have to update the cost price of the user card product each time.

2. Deliver the new car:

• CREDIT to INVENTORY for $25k (decrease the value of Inventory)
• DEBIT to INTERIM DELIVERED for $25K (hold the counterpart for the decrease until we have an Invoice for it)

3. Create the Invoice:

• DEBIT to COST OF GOODS for $25K (the cost price of the NEW CAR)
• CREDIT to INTERIM DELIVERED for $25K (unhold the counterpart from 2)
• DEBIT to A/R for $15k ($30k sales price of the NEW CAR minus the $15k cost price of the USED CAR)
• CREDIT to SALES for $30K (for the sales price of the NEW CAR)
• DEBIT to INTERIM RECEIVED for $15k (unhold the counterpart from 1)

Note: on your Invoice, the first line is the line for the new car which is booked to the sales/revenue account.  Ask Odoo to create the Invoice from the Sales Order, automatically, in the standard way. Then manually add a second line to deal with the used car component. The second line would not have a product, and be booked to the stock interim account used in the first step, to clear the counterpart (and it would have a negative price, to reduce the amount due from the Customer).