The merit shouldn't be returned to sender if the post is deleted. It should just be destroyed. If a post is gone then so should the merit. People could just abuse it or delete the post to give the merit back for whatever reason.
as far as i know only moderator can look into deleted posts (cmiiw) so it will be hard for people to help monitoring other members activity. There should be an automated archive for deleted posts that have merit on it.
Only admins can see deleted posts and merit abuse isn't something they're going to waste time on.