Ah, yes, I should have assumed someone would dredge up Scott v. Sandford, 60 U.S. 393 (1856).
Chief Justice Taney did not say that Congress could not make a law freeing slaves.
Indeed, the Court was not ruling on the issue. The Court was ruling whether a) Dred Scott, a slave, was a citizen of the United States entitled to sue in federal court; b) Dred Scott had become a free person by travelling into the federally organized territory of Upper Louisiana; and c) Dred Scott had become a free person by travelling to the territory of the State of Illinois.
As to a), the Court ruled that slaves were not citizens under the Constitution
As to c), the Court ruled that he did not, because of the prior holding of the Court in Strader et al. v. Graham, 10th Howard, 82 (sorry, I can’t find the modern citation).
As to b), the Court ruled that he did not. In so ruling, the Court determined that the unorganized territory was governed by the limitations on federal power placed in the Constitution, and that the Constitution conferred no power on Congress to eliminate the property rights of a person who transported that property from a state in which the property was legally owned to a federal territory. This ruling was not grounded solely in the provisions of the Fifth Amendment.
Indeed, the quote from the opinion does not support the proposition offered by nebuli. It simply states that the property bond properly formed elsewhere cannot be severed merely through transportation from a state into federal territory without violating due process.
Which leaves us exactly where we started; Congress might have been prevented under Article I from freeing slaves because it wasn’t the exercise of a specifically granted power, Congress was prohibited from preventing the interstate commerce of slavery for 20 years, impliedly permitting the ownership of slaves for those 20 years, Congress could not deprive the owner of his property without due process, and due process did not include taking away that ownership status simply because the property entered the federal territory or another state.