Function to get content by ID

Instead of using get_posts, which you would use if you wanted to retrieve multiple posts in a loop, you should use get_post, which only retrieves one post by an ID. There is also a built-in excerpt so you might want to go with retrieving post_excerpt.

function get_the_excerpt_id($post_id) {
    $find = get_post($post_id); 
    $excerpt = $find->post_excerpt;

    $excerpt = strip_tags($excerpt);
    $output = substr($excerpt, 0, 100);

return $output;