My husband works as a truck driver and when he is paid his check stub shows gross pay and then deductions and there is a deduction for per diem. Why is this a deduction.

I would call up the payroll dept and speak to them as the per diem pay should not be deducted, it should be added. When I did a trucking company's payroll, this item was not deducted. Maybe the company could clarify this for you and correct their error. Per diem pay is just that - pay.