How to add a validation on the expense item amount

Hi Experts

we have requirements to implement a validation on the expense item based on the employee grade, for example; the employee with grade 5 cannot submit a schooling allowance more than SAR 40K based on the company policy, so we need a validation on the amount field based on the amount and grade to show an error message with deviates from the policy.

Any idea how we can do this?