There can be more than one primary. To do what you want your new zone would also have to be primary. All you are wanting to do is override a single
www.foreigndomain.com, so create a primary zone on your AD DNS called
www.forigndomain.com, add a single "A" record with the host field blank and the ip address of your internal server.
That way when users enter
http://www.foreigndomain.com, DNS queries your AD server who recognizes that it is one of his own domains and sends the ip of your internal server. Any other business you do with foreigndomain.com will resolve from their public dns servers.
Good Luck