When you sell a vehicle you will want to record the details of any PX, and you will want it to be linked to the vehicle you are selling, you do this from the 'Summary' screen by pressing the button as shown:

When you press it a new vehicle record screen will be shown in which you can record the details of the PX vehicle.  Once you save this new record, you will be returned to the vehicle record of the vehicle that you are selling and the PX total will be updated for you.  You can also manually add a PX value in the fist column if you wish.

This screen lists all linked PX vehicles and allows you to view the record or delete the link (not the record)