Warning alert for over budgeting

Summary

How to trigger an non blocking alert to display when the manager allocate over budget

Content

Hi all,

we have checked the option "Allow over budget when allocating compensation" in order to can overwhelm the budget amount.

The requirements is to trigger an alert (not blocking alert, not error, just warning) to display to manager that the SUM of 
compensation amount proposed for his reports exceed the budget.

How is it possible accomplish this requirement?

The conditional logic that trigger the alert should be something like:
- SUM(amount component 1) > Budget component 1

OR 

- Remaining budget component 1 < 0

Kind Regards,
Alessandro