Is there a way to get a file URL from the relationship ACF field?

$posts = get_sub_field('download_existing');
     $existing_link_url="";
     foreach ((array)$posts as $link_posts) {
       $existing_link_url = get_field("your_field",$link_posts->ID);
     }

This will overite in the loop the $existing_link_url so you may want to save it.