It's not international, but from a Southern point of view, it was nearly so:

Charles A. Stovall traveled from Mississippi to California in late 1857, accompanied by his slave, Archy Lee. In January 1858, Stovall decided to return to the South, and he planned to take Lee with him as his property. Lee, who was either seventeen or eighteen years old at the time, escaped and found supporters who helped him declare that he was a free man because California did not allow slavery.

This resulted is what may have been the only California legal contest testing the federal Fugitive Slave Act, passed by Congress in September 1850. That law asserted that slaves who escaped to free states remained the property of their former masters and that their return was required by law. Stovall maintained that his slave had escaped and that the free status of California was irrelevant because of the Fugitive Slave Act. Lee argued that he had not escaped to California, but rather Stovall had taken him and settled in a free state where slavery was illegal.

A California court decided in favor of Lee, but upon appeal, Justice Peter H. Burnett, former California governor, wrote a majority opinion for the California State Supreme Court, asserting that the ambiguity of the case should result in a decision in favour of Stovall. On March 5, 1858, Stovall attempted to escape with Lee. Local abolitionists discovered the plan and had Stovall arrested for kidnapping, in defiance of the ruling of the California Supreme Court. Later that month, the U.S. District federal court in San Francisco overturned the state court, asserting that Lee was, indeed, a free man.

Stovall appealed that ruling to the U.S. Commissioner, claiming a violation of his prerogatives as defined by the Fugitive Slave Act of 1850, but the commissioner reaffirmed the freedom of Lee, pointing out that Lee had not crossed a state line to escape.

This had an odd epilogue in the form of a duel:

On August 21, 1858, William I. Ferguson and George Pendleton Johnston fought a duel on Angel Island in the north end of the San Francisco Bay. Both men were involved in the politics of the state: Kentucky-born Johnston had been a member of the assembly, and he served as the United State Court Commissioner, taking the side of an escaped slave in an important test of the Fugitive Slave Act of 1850. Ferguson was a native of Pennsylvania who was elected to the California Senate, representing the anti-immigrant, nativist movement known as the Know-Nothings. He subsequently renounced the Know-Nothings.

With the heightened political animosity during the election of 1858, Ferguson attracted attention, in particular because of his sense of humor, which was frequently accentuated by drinking at local saloons. In mid-August, while drinking with Johnston, Ferguson told a joke which derided friends of Johnston. This precipitated a fight and weapons were drawn, but friends parted the two men. The following day, Johnston sent a friend to Ferguson demanding an apology, but Ferguson refused, inspiring a challenge for a duel, which Ferguson accepted.

The duel ensued with a series of three shots exchanged without effect. With each round, the two men took a step closer. Although both combatants agreed that they would only fire three rounds, Johnston persisted in insisting upon an apology. Ferguson again refused, so they two men fired a fourth round, during which Ferguson's thigh was shattered by a shot. Both men shook hands, satisfied that their disagreement had been resolved, but Ferguson's condition was grave.

Doctor's subsequently advised Ferguson that his only hope of survival would be to amputate his leg, which he refused. His wound festered, and so on September 14, 1858, he agreed to an amputation, but he died during the operation. Johnston was tried, but he was acquitted because of a successful argument that Ferguson would have survived had he agreed to an amputation immediately after being wounded.

The image of an affidavit from Stovall is available at the National Archives website.

Sources: Rudolph M. Lapp, Archy Lee: A California Fugitive Slave Case (San Francisco: The Book Club of California, 1969); William E. Franklin, "The Archy Case: The California Supreme Court Refuses to Free a Slave," Pacific Historical Review, 32:2 (May 1863) 137-54; Sacramento Daily Union (August 23, 1858) 2:3, and (September 17, 1858) 1:3-7; Oscar Tully Shuck, History of the Bench and Bar of California (Los Angeles: Commercial Printing House, 1901) 240-45.

While it is pre-United States, and slightly different on the facts (the slave didn't decline to return but escaped) there is one particularly famous example; Charles Stewart and James Somerset.

[Disclaimer; I have a background in law not history, so I welcome any additions or corrections to this.]

Charles Stewart (or Steuart) was a Scottish-born Virginian. By the 1760s he was a customs officer in Boston and had bought James Somerset as a slave. In 1769 Stewart travelled to Great Britain with Somerset. While in England Somerset met with representatives of the abolitionist movement (and freed or free black people) and in 1771 ran away from Stewart. Stewart was able to capture Somerset and had him imprisoned on a ship so he could be transported back to the Americas (specifically Jamaica) to be sold off. Somerset's godparents (backed by an abolitionist) made an application for a writ of habeas corpus against the ship's captain, asking for the legal justification for detaining Somerset.

The case was heard before the Lord Chief Justice, Lord Mansfield, on 14 May 1772, and Stewart relied entirely on his ownership of Somerset as a slave. Lord Mansfield gave his opinion on 22 June 1772, finding that the ship's captain had no grounds, at common law, to remove Somerset from England by force, and ordered that Somerset be released.

While the precise scope of his ruling is debated (Lord Mansfield later suggested it only covered the power to remove someone from the country), the judgment rested on a finding that English common law did not recognise slavery as slavery was so "odious" and serious it required positive law (e.g. Act of Parliament).

This case resulted in the effective abolition of slavery in England (and later, Scotland), but did not comment on the legality of slavery in other places under British jurisdiction (including the North American, British-controlled colonies).

Somerset v Stewart (1772) 98 ER 499