If you’re being reimbursed by your employer, you’ll need to provide an expense report, and return any excess funds.  If the place, time and purpose are verified, per diem plans or mileage allowances may be used in place of detailed accounting. Record your expenses as soon as possible.

The IRS only requires detailed accounting in these cases if: you aren’t fully reimbursed, or if you fail to return excess reimbursement.