The Margaret, Alias Carlos Fernando Haley case was a dispute between the United States and Spain over ownership of a vessel. The claimant argued that it had been purchased from Spanish owners in 1820 by an American citizen who then sold it to another American citizen in 1821. However, the US government argued that since the ship had been built in Spain and registered there before being sold to Americans, it should be considered as property of Spain under international law. The Supreme Court ultimately sided with the US government's argument and held that although title may have passed from one owner to another within America, this did not change its status as Spanish property under international law.

In this case, the Supreme Court was asked to decide whether a slave named Margaret (alias Carlos Fernando Haley) should be freed. The majority opinion held that she should not be freed because her owner had taken her from Virginia to Maryland and back again in violation of the laws of both states. However, Justice Johnson dissented from this decision and argued that Margaret's status as a slave should not depend on which state she happened to reside in at any given time. He reasoned that since slavery is an institution based upon force rather than consent, it cannot exist outside its own jurisdiction; therefore, when Margaret left Virginia with her master's permission - even if he did so illegally - she effectively ceased being his property and became free. Furthermore, Johnson noted that while Congress had enacted legislation prohibiting slaves from leaving their home states without written permission from their owners or other legal authority figures, such laws were unconstitutional under Article IV Section 2 of the Constitution which guarantees citizens "the Privilege of the Writ of Habeas Corpus." Thus according to Justice Johnson's dissent, Margaret was entitled to freedom regardless of where she traveled with her master due to these constitutional protections against involuntary servitude.