in The Netherlands the transfer of ownership is done with an invoice, as a deed of ownership.
In the small print it normally says that ownership transfer is withheld when the item has not been paid in full.
In other words, the process of transfer is completed only when the invoice is paid in full.
Delivery of the article is not part of transfer of ownership.
In the Netherlands he would now have a watch in possesion which is officially yours. But refusing to hand it over.
In the US this might be slightly different, even if I do not think so, but lawyers please chime in.