Short answers, pulled from the story.
Article IV, Section 2, Clause 3 of the United States Constitution requires that such a person be returned to their master. This legal language deliberately avoids using the words slave and slavery throughout the document.
Lord Mansfield ordered that the individual was a free person in 1772 when he ruled they could not be legally returned absent local custom or positive legislation. This decision established a precedent where judges were bound by English law to ignore prior legal status under foreign laws.
Two South Carolina delegates Charles Pinckney and Pierce Butler proposed on the 28th of August 1787 that fugitive slaves should be delivered up like criminals. James Wilson of Pennsylvania and Roger Sherman of Connecticut originally objected to this provision before it was passed without debate the next day.
An effort to repeal this clause failed in 1864 when the House voted 69 for repeal and 38 against which fell short of the two-to-one vote required to amend the constitution. The failure occurred during the Civil War while the Constitution of the Confederate States contained a much more rigid form of the clause.
The passage of the Thirteenth Amendment abolished slavery except as punishment for crime rendering the clause mostly moot due to the abolition of slavery itself. The U.S. Supreme Court stated in United States v Kozminski 487 U.S. 931 943 1988 that not all situations involving compelled labor violate the amendment.