Your per diem is only non-taxable when the assignment you are on is expected to last less than one year. Once you find out that your assignment will last beyond that point, then the per diem amounts are subject to tax. Amounts you have been paid up to this point will continue to be non-taxable.
As far as moving to another location for a month, you may get away with it as far as how your employer handles the taxability of these payments, but if you are ever audited by the IRS, they would likely end up disallowing you to claim this as non-taxable income
There is nothing written in stone about this issue, and the same question comes up from a number of people who pass through this forum, who are trying to accomplish the same thing as you are.
But the IRS is well aware of this. They would likely take the position that this move was made in an effort to circumvent the rules
, and say that you were made aware this position would no longer be less than one year. It just depends on if you are willing to risk having this surface in the event of an audit.
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Thank you nicksie.