Technically, there is no notification that alerts an admin when users are added specifically using the Manage Users page. However, you could add a cc to the enrollments message and that could send a notification when someone is enrolled through the Manage Users page. You just wouldn't be able to differentiate how they were enrolled.
The thing missing is a confirmation notification that people who purchased enrollments had them assigned. That is currently unavailable. This would be more important if you were selling live classes where capacity might be an issue, but with on-demand courses, The best way to check enrollment and unassigned purchases would be to go to the Admin purchases page and look at the course. There, it would tell the admin who was enrolled and how many unassigned purchases were left just the same as if it were the user on their Manage Users page.