General Deduction with a monthly limit

The title pretty much says it all - we are looking for a solution to cap a general deduction based on a monthly amount. An example is a parking deduction.

We have employees with multiple jobs, and in some cases one job is monthly and another is biweekly. So, when we enroll the employee in a parking deduction, it attempts to deduct both on their monthly check and their biweekly check. By having a monthly limit, we can ensure that the monthly deduction would not deduct if they had already had the deduction on a prior biweekly check.