If a U.S. citizen conspires to have another U.S. citizen murdered but the actual killing takes place in a foreign country, can the accessory to the murder be pardoned by the President?
He can, to the extent that there is a federal charge involved. There is no limit to the power to "grant reprieves and pardons for offenses against the United States", but there is no power to grant reprieve or pardon against a US state or foreign country.
In other words, a presidential pardon would prevent the person from being tried or punished by the US federal government. It would not prevent them from being tried or punished by a state or foreign government.