As the equitable title holder you are invoicing him for the use of the vehicle?
You don't own anything you register as far as I'm aware. It's the titles... Possession is 9/10th of the law, title is the other 10th...
Have you a copy of the invoice you could post up?
Printed copy · Anthology Volume 6, page 717Printed copy from The Tender For Law Anthology, Volume 6, page 717 (printed 13 June 2014). Added by this site; not part of the Facebook export.