Custom URL routes

All internal rewrite rules must point to index.php. This isn’t a theme file, it’s the main bootstrap file in the root of your WordPress install. So, your rule should look like:

add_action( 'init', 'wpse26388_rewrites_init' );
function wpse26388_rewrites_init(){
    add_rewrite_rule(
        'properties/([0-9]+)/?$',
        'index.php?pagename=single-deal&property_id=$matches[1]',
        'top'
    );
}

You can then assign the deal.php template to your single-deal page in the admin interface, or you can use the page_template filter if you want to apply conditional template loading, like for example only if your property_id query var is set.