Well, I started on that path with gradual compensated emancipation by the 1840s in "Created Equal," so you could probably speed things up a few years with a 11796 POD like I used; what touches it off is a very unusual war between the states that was just that - several states fighting each other before federal troops are sent in (think the Toledo War but down South & larger) Theoretically that could come earlier.
Maybe my POD somehow leads to the boll weevil coming and destroying the cotton fields, making it economically untenable; could it come a century early? I'm not sure when it came in other places. Finally, you need a place for them as noted, but the area held by the U.S. still seems massive, and some could aruge that the freed slaves would be the best colonists as buffers against the natives.
Earlier PODs, of course, can also apply - Virginia abolishing it in the 1780s, perhaps as mentioned no ARW. I'm not sure if you can get one post-1796, though Adams winning in 1800 is a thought as his pushing for admission of Louisiana Territory as totally free is a big step. I just don't know if his winning in 1800 is that easy without a Jefferson win in 1796.