Lincoln had no constitutional authority to free those slaves in States that remained in the Union.
Did he have the Constitutional authority to free them in states in rebellion? Under what auspices, if so?
Where exactly was slavery written into The Constitution?
He had no constitutional authority to free any of them. In fact, the Constitution explicitly required that they be returned to the person to whom their labor is legally due under the laws of the state where they were held.
But we just gloss over Constitutional "law" when we don't like the law.