Northern slaves? The Emancipation Proclamation freed all slaves in the United States. Even before that the northern states had outlawed slavery (maybe not in reality). I am not familiar with northern slavery. What is your source?
The president had no power whatsoever to interfere with the institution of slavery during the normal administration of the government. President Lincoln worked hard for passage of the 13th and 14th amendments. He freed the slaves in the insurgent area under the war power granted the president in the Constitution.
Lincoln wrote two famous letters on this subject.
To James Conkling:
"But to be plain, you are dissatisfied with me about the negro. Quite likely there is a difference of opinion between you and myself upon that subject. I certainly wish that all men could be free, while I suppose that you do not. Yet I have neither adopted nor proposed any measure, which is not consistant even with your view, provided you are for the Union. I suggested compensated emancipation; to which you replied you wished not to be taxed to buy negroes. But I had not asked you to be taxed to buy negroes, except in such way, as to save you from greater expense, to save the Union exclusively by other means. You dislike the emancipatio proclamation; and perhaps, would have it retracted. You say it is unconstitutional--I think differently. I think the Constitution invests the commander in chief with the law of war, in time of war.
The most that can be said, if so much, is, that slaves are property. Is there--has there ever been--any question that by the law of war, property, both of enemies and friends, may be taken when needed? And is it not needed whenever taking it helps us, or hurts the enemy?
....but the proclamation, as law, either is valid, or it is not valid. If it is not valid, it needs no retraction. If it is valid, it can not be retracted, any more than the dead can be brought to life....The war has certainly progressed as favorably for us, since the issue of the proclamation as before. I know as fully as one can know the opinions of others that some of the commanders of our armies in the field who have given us some of most important successes, believe the emancipation policy and the use of colored troops, constitute the heaviest blow yet dealt the rebellion, and that at least one of those important successes could not have been achieved when it was but for the aid of black soldiers....I submit these opinions as being entitled to some weight against the objections, often urged, that emancipation, and arming the blacks, are unwise as military measures, and were not adopted, as such, in good faith. You say you will not fight to free negroes. Some of them seem willing to fight for you; but no matter. Fight you then, exclusively to save the Union...
negroes, like other people act upon motives. Why should they do anything for us if we will do nothing for them? If they stake their lives for us, they must be prompted by the strongest motive--even the promise of freedom. And the promise, being made, must be kept....peace does not appear as distant as it did. I hope it will come soon, and come to stay; and so come as to worth the keeping in all future time. It will have then been proved that, among free men, there can be no successful appeal from the ballot to the bullet; and that they who take such appeal are sure to lose their case, and pay the cost. And then, there will be some black men, who can remember that, with silent tongue, and clenched teeth, and steady eye, and well-poised bayonet they have helped mankind on to this great consumation; while, I fear, there will be some white ones, unable to forget that, with malignant heart, and deceitful speech, have strove to hinder it. Still let us not be over-sanguine of a speedy final triumph. Let us be quite sober. Let us dilligently apply the means, never doubting that a just God, in his own good time, will give us the rightful result."
And to A. G. Hodges:
"You ask me to put in writing the substance of what I verbally said the other day, in your presence, to Governor Bramlette and Senator Dixon. It was about as follows:
"I am naturally anti-slavery. If slavery is not wrong, nothing is wrong. I can not remember when I did not so think, and feel. And yet I have never understood that the presidency conferred upon me an unrestricted right to act upon this judgment and feeling. It was in the oath I took, that I would, to the utmost of my ability, preserve, protect and defend the Constitution of the United States. I could not take the office without taking the oath. Nor was it my view that I might take an oath to get power, and break the oath in using the power. I understood too, that in ordinary civil administration this oath even forbade me to practically indulge my primary abstract judgment on the moral question of slavery. I have publically declared this many times, and in many ways. And I aver that, to this day, I have done no official act in mere deference to my abstract judgment and feeling on slavery.
I did understand however that my oath to preserve the constitution to the best of my ability, imposed upon me the duty of preserving by every indispensible means, that government--that nation--of which that constitution was the organic law. Was it possible to lose the nation, and preserve the constitution? By general law life and limb must be protected; yet often a limb must be amputated to save a life; but a life is never wisely given to save a limb. I felt that measures, otherwise unconstitutional, might become lawful, by becoming indispensible to to the preservation of the of the Constitution, through the preservation of the nation. Right or wrong, I assumed this ground, and now avow it...
When in March, and May and July 1862 I made earnest, and succcessive appeals to the border states to favor compensated emancipation, I believed the indispensable neccessity for military emancipation and arming the blacks would come, unless averted by that measure. They declined the proposition; and I was, in my best judgment, driven to the alternative of either surrendering the Union, and with it the Constitution, or of laying strong hand upon the colored element. I chose the latter. In choosing it, I hoped for greater gain than loss; but of this, I was not entirely confident. More than a year of trial now shows no loss by it in our foreign relations, none in our home popular sentiment, none in our white military force, no loss any how or any where. On the contrary, it shows a gain of quite one hundred and thirty thousand soldiers, seamen and laborers. These are palpable facts, about which there can be no cavilling. We have the men; and we could not have them without the measure.
And now let any Union man who complains of the measure, test himself by writing down in one line that he is for subduing the rebellion by force of arms; and in the next, that he is for taking these hundred and thirty thousand men from the Union side, and placing them where they would be but for the measure he condemns. If he can not face his case so stated, it is only because he can not face the truth.
I add a word which was not in the verbal conversation. In telling this tale I attempt no compliment to my own sagacity. I claim not to have controlled events, but confess plainly that events have controlled me. Now, at the end of three years struggle the Nation's condition is not what either party, or any man devised, or expected. God alone can claim it. Whither it is tending seems plain. If God now wills the removal of a great wrong, and wills also that we of the North as well as you of the South, shall pay for our complicity in that wrong, impartial history will find therein new cause to attest and revere the justice and goodness of God."
Quoted from "Lincoln; Speeches and Writings, 1859-65, Library of the Americas, D. Fehrenbacher, ed.