Did he have the Constitutional authority to free them in states in rebellion? Under what auspices, if so?
The President's position as Commander in Chief gave him the power to seize the property of those making war on the United States. Lincoln was under pressure to take this action as soon as Fort Sumter was attacked. He hesitated, for fear of the reaction in the border states. Congress passed Confiscation Acts, urging that slaves owned by Confederates be freed. US Army units in the south were swarmed with runaway slaves.
Commander in Chief of the Army and Navy. The freeing of slaves in the South helped the U.S. Governments cause and impeded the Confederate cause.