Due to the Heart Act foreign workers who have owned a green card in eight of the last 15 years and choose to relinquish it will be subject to the Expatriation Tax, which taxes unrealized gains above $600,000, anywhere in the world. However this will only apply to those people who have a federal tax liability greater than $139,000 a year or have a worth of more than $2 million or have failed to certify to the IRS that they have been in compliance with US federal tax obligations for the past five years.
If the green card is not relinquished then the holder is subject to double taxation when living or working outside of the United States, whether or not within their home nation, although double taxation may be mitigated by foreign tax credits.