Lincoln had absolutely zero constitutional authority to free slaves in states that were not in rebellion against the United States. However, due to the confiscation act passed by congress in August 1861 he did was given the authority to have the military confiscate any property that would help the insurrectionists. This included slaves. Here’s the text of the law that covered slaves.
SEC. 4. And be it further enacted, That whenever hereafter, during the present insurrection against the Government of the United States, any person claimed to be held to labor or service under the law of any State, shall be required or permitted by the person to whom such labor or service is claimed to be due, or by the lawful agent of such person, to take up arms against the United States, or shall be required or permitted by the person to whom such labor or service is claimed to be due, or his lawful agent, to work or to be employed in or upon any fort, navy yard, dock, armory, ship, entrenchment, or in any military or naval service whatsoever, against the Government and lawful authority of the United States, then, and in every such case, the person to whom such labor or service is claimed to be due shall forfeit his claim to such labor, any law of the State or of the United States to the contrary notwithstanding. And whenever thereafter the person claiming such labor or service shall seek to enforce his claim, it shall be a full and sufficient answer to such claim that the person whose service or labor is claimed had been employed in hostile service against the Government of the United States, contrary to the provisions of this act.
APPROVED, August 6, 1861[5]
There were no insurrectionists, except Lincoln and his merry band of thugs. Lincoln had already initiated an unconstitutional war on a sovereign nation, and violently usurped all other powers that didn't belong to the executive branch, including habeas corpus, before that Act was even introduced.
I tend to be suspicious of all written history (clinging to the adage, "verify, then trust";) but sometimes little gems sneak through to clarify certain anomalies, such as Lincoln's handling of General McClellan:
"McClellan authorized seizure of personal property if necessary for military purposes, but specified that detailed records would be kept and receipts issued in all cases, regardless of a given civilian's loyalty. He continued, 'The idea that private property may be plundered with impunity is perhaps the very worst that can pervade an army, and added, 'The general commanding takes this occasion to remind the officers and soldiers of this army that we are engaged in supporting the Constitution and laws of the United States and in suppressing rebellion against their authority; that we are not engaged in a war of rapine, revenge, or subjugation; that this is not a contest against populations, but against armed forces and political organizations; that it is a struggle carried on within the United States, and should be conducted by us upon the highest principles known to Christian civilization.'" [Mark Grimsley, "The Hard Hand of War: Union military policy toward Southern civilians, 1861-1865." Cambridge University Press, 1997, p.89]
Perhaps General McClellan was too civilized – too endeared to the Constitution and Holy Bible -- for Lincoln to keep him around. There is also the case of Major General Butler:
"[General] Butler was impatient with troops who committed depredations. 'The Volunteer troops seem to have adopted the theory that all property of the inhabitants was subject to plunder,' he wrote Scott shortly after assuming command at Fort Monroe."I have taken the most energetic measures to correct this idea and prevent plundering. There are some few flagrant instances, which can admit neither of palliation nor of justification." But he did not hesitate to retaliate against secessionists who trifled with the Union government." [Ibid. pp.52-53]
Butler also ended up losing his command. Just saying . . .
No matter how historians have spun those particular events, they have also left no doubt that Lincoln was an exceptionally brutal person who preferred the terrorist commanding general over the traditional one.
Mr. Kalamata