301-74.13—May we include conference administrative costs in an employee's per diem allowance payment for attendance at a conference?

No. Per diem is intended only to reimburse the attendee's subsistence expenses. You must pay conference registration fees separately, either directly or by reimbursing employees who pay such expenses and submit travel claims.

CFR

301-74.13—May we include conference administrative costs in an employee's per diem allowance payment for attendance at a conference?