It means that FBR doesn't know that you are the owner of car, as they do not have any system through which they get details of car regn, and send the notices to owners.
No. Those receipts would not serve the purpose. How will negate their question that "you have purchased these dollars from open market and then encashed them and deposited in your account"?
Now, it is not likely that you will receive any notice from FBR.
Even if they somehow manage to get info regarding your ownership, and send you notice, then consult any tax consultant for your wealth preparation, and show as many cash as he can in your wealth. That is dependent on your age, whats you age right now?