Depending on the length and terms of the US assignment, tax relief may be available under the provisions of a bilateral tax treaty between the US and the home country. Generally, treaty relief for compensation is only available if the individual is not present in the US for more than 183 days during that year and the compensation is paid and borne by an offshore, (ie a non-US) entity. It is critical that the treaty provisions of each particular country be examined.
In addition to treaty relief there is a 'closer connection' statue in US tax law that allows individuals who have met the substantial presence test (described above) but have not been present in the US for 183 days in the current year to elect a ‘closer connection’ to a foreign country and thus become a US tax non-resident.
Non-ECI (which typically includes investment income such as interest, dividends, rents and royalties) is taxed to the extent that it is deemed to be derived from US sources. Non-ECI is taxed as gross income, (ie no deductions are allowed), generally at a flat rate of 30%, but if the non-resident alien is resident in a country with which the US has a tax treaty, a lower rate may apply.
For those individuals coming on assignment to the US for one year or less, there are allowed exclusions from income for certain business travel expenses such as meals, temporary lodging, and transportation. These expenses must be reasonable in nature and reimbursed under an accountable plan (ie employee submits request with receipts) or a flat per diem rate up to a certain limit adjusted by the IRS.