You are correct that the Summary values are not calculated until after the record is Saved so the form Rules will never fire.
The suggestion I have though is to send an instant email notification to the user who just created the parent record to let them know they made a mistake. That way they can correct their mistake before payroll has a chance to review it. Generally, users are trying not to make mistakes and if given an opportunity they will correct them. You can also place a big red warning message on the record itself so that when the user saves the record not only will they receive an instant email beep but they will also see a big red error message.
