Can you please provide some support for that contention? I'm no expert on the Civil War era, but last I knew, at the time of the war most of the States of the Union were "free" states, that is, did not permit slavery.
If that's not the case I'd appreciate knowing the source(s) of your data. Thanks.
My evidence is simply observational and a good understanding of human nature. When the South tried selling its cotton to Europe, for the world price, instead of the domestic priced enforced upon them by the Northern textile industrialists, they proceeded to block the South from doing so during the cotton blockade.
I know the South seceded over slavery, because what choice did they have if they understood that the North would agitate for the end of slavery but not compensate the South by paying a higher price for cotton? This would have bankrupted the Southern plantations. Being bankrupt, the Southern plantations would have to sell their land for whatever somebody was willing to pay. This would have opened the opportunity for the North to own the land and continue to grow the cotton at the lowest price possible to feed their textile mills.
I’m not claiming that this is what was happening, but it would fit with everything else that was happening.