Custom notification for contributors when posts are scheduled

Instead of rely on global $post, you should use the post object passed to function via the posts status transition:

add_action('pending_to_future', 'scheduledNotification');
add_action('approved_to_future', 'scheduledNotification');

function scheduledNotification( $post ) {
   $author = get_userdata($post->post_author);
   // the rest of your function here
}