Retroactive Amendment of Travel Orders To Reduce Per Diem Allowance

An opinion was requested concerning an employee's claim for additional per diem for the first 30 days of his 90-day temporary duty assignment. Travel orders authorized per diem at a flat rate of $33 for the 30 days contrary to an agency instruction setting per diem at "$14 plus lodgings not to exceed $33 for the first 30 days." The certifying officer properly reduced the allowable per diem retroactively to the amount prescribed by the instruction. The general rule against retroactive modification applies only to orders issued within the scope of the authority of the authorizing official.