While I am sure that the logic of what I am about to post is too simple, I think it should get you started.
Your terminology is odd but what it seems you want is to display the “post before the more” part of the post body. To do that, you need to break the post content into “post before the more” and “post after the more”. There is a function for that.
$pieces = get_extended($post->post_content);
// var_dump($pieces); // debug
echo apply_filters('the_content',$pieces['main']);
Inside a Loop, that should work just fine. In other contexts you may need something other than $post->post_content.
Excerpts, manually or automatically created, will not come into play so you may need to include that in the logic.