Lincoln had fought the war to retain the Union.
That if he could be certain to do so by allowing slavery he would do so, if he could be certain to do so by freeing slaves he would do so and if he could be certain to do so by other measure he would do so.
But he also had to appease the Black Republican Wing during the iffy period of 1862, so he used the 'win' at Antietam to launch it.