Limit the annual Leave

Hello,

I need to create a vacation type and then set a maximum limit of hours for this vacation during the calendar year. What i did is that i created a pay type for this vacation type and then i created an accrual because based on the this accrual i calculate some deduction. What i need to do is to find a way that when trying to enter the pay type of this leave with a number of hours more then the annual permitted the system stop the user from entering it.

Ex: If the annual limit is 168 Hours, and the Employee accrues 14 hours per month