41 CFR § 301-11.200
Under what circumstances may my agency prescribe a reduced per diem rate lower than the prescribed maximum?
June 9, 2020
Under the following circumstances:
(a) When your agency can determine in advance that lodging and/or meal costs will be lower than the per diem rate; and
(b) The lowest authorized per diem rate must be stated in your travel authorization in advance of your travel.