First it depends on whether the plan is an accountable per diem plan or not. The IRS
says that it is, if it meets three requirements.
To be an accountable plan, your employer's reimbursement
arrangement must include all of the following rules
"Your expenses must have a business connection — that is, you must have paid or incurred deductible
expenses while performing services as an employee of your employer.
You must adequately account to your employer for these expenses within a reasonable period of time.
You must return
any excess reimbursement or allowance within a reasonable period of time."
If you don't meet the conditions for an accountable plan, the plan is called "non-accountable," and all reimbursements, not just "excess" reimbursements, may be subject to tax..... to you the employee.
On the foreign issue all U.S. citizens must pay tax on all worldwide income. And if you are working for a US
company while traveling overseas you pay income on that as well.
So it' really the meeting of the accountable plan conditions that determine taxation
Hope this helps
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