It already was in the law. Here is what an 1856 Law Dictionary says:
SLAVE. A man who is by law deprived of his liberty for life, and becomes the property of another.
2. A slave has no political rights, and generally has no civil rights. He can enter into no contract unless specially authorized by law; what he acquires generally, belongs to his master. The children of female slaves follow the condition of their mothers, and are themselves slaves.
3. In Maryland, Missouri and Virginia slaves are declared by statute to be personal estate, or treated as such. Anth. Shep. To. 428, 494; Misso. Laws, 558. In Kentucky, the rule is different, and they are considered real estate. 1 Kty. Rev. Laws, 566 1 Dana's R. 94.
4. In general a slave is considered a thing and not a person; but sometimes he is considered as a person; as when he commits a crime; for example, two white persons and a slave can commit a riot. 1 McCord, 534. See Person.
Show me the actual law passed by the US Congress, that the 1856 law dictionary quotes.
It doesn't exist. In typical legalese we are told of the law but not shown the law because slaves were basically enslaved by common law practices. Like a man and a woman living together are these days considered a common law, marriage it is a marriage in every sense of the word but not legally formalized.
So was slavery, with the exception of prisoners, there existed no laws on the books that could legally confine, indenture or enslave anyone with a legal reason for doing so. The whole system of slavery was based upon the ignorance and fear of those enslaved created by their masters.
Ask yourself this one question why were there so many cases dealing with runaway slaves and yet no cases tried on the ultimate legality of slavery?
Because everytime it was attempted it was easier to "free" the slave then to try the legality of slavery, those cases dealing with runaway slaves were the only way to get the issue into the courts and tried.