And should those states have returned to the Union, they would no longer have been in rebellion and the slaves would have returned to their former status: property.
Sure, that was the last hope of Confederate negotiators with Lincoln at war's end, early 1865 -- they would agree to surrender if Lincoln would agree to preserve slavery.
But Lincoln would have none of it, and his response was to rush through passing the 13th Amendment, to make dead certain slavery could never again raise its ugly head.