here is something to help out
The term “Northern States” is not a well defined one. Are the border slave states that remained in the Union during the Civil War “Northern States”?
Clearly all slave states in the Union (Maryland, Missouri, Delaware, and Kentucky, and the District of Columbia) benefited because they had slaves (though the slaves themselves did not ‘benefit’).
Even in some of the states that had abolished slavery not all the former slaves were really free, as the legislation still bound them to their former master in manner like indentured servitude ("apprenticed for life" ). The states where this was most prevalent were New Jersey and Delaware, which abolished slavery without freeing the slaves from their masters in the 1840s, the last of these “apprentices” were not freed until 1865 by the ratification of the 13th Amendment. It is interesting that New Jersey and Delaware were the last Union states to ratify the 13th Amendment, and only did so after it had gone into effect and the “apprentices” had all been freed. Delaware did not ratify it until 1901!
The North had textile mills that consumed cotton (about 15% of the southern cotton production fed Northern mills) and so benefited in that sense, the same as did Great Britain which bought nearly all of the rest of the cotton crop.
But the biggest benefit to the North were the capitalists who invested in slavery.
The westward rapid expansion of slave plantations was financed with debt secured by the bodies of slaves. Investors preferred that the debt be secured by slaves, rather than land, for two reasons - the slaves were more valuable than the land they worked, and unlike land they were liquid - they could be bought and sold anywhere that had slavery and were in high demand. The source of this capital financing slavery was principally Britain, but Northern banks and investors held a large part of the debt as well.
If a plantation owner went bankrupt (cotton plantations were an extremely profitable business, but in any boom among those seeking to get rich are poor businessmen and managers) and when the debt was foreclosed the bank/private investor in the North and in anti-slavery Britain became slaveholders until they liquidated their “asset”.
The other way the North (and to a lesser extent Britain) profited from slavery was in financing slaving voyages. Although the importation of slaves from outside the U.S. was banned in 1807, and Britain began interdicting slaving ships in that same year, the slave trade continued to the non-British Caribbean and South America. Most of the slaving vessels that engaged in this were built in northern U.S. shipyards and flew U.S. flags, using it for protection from boarding.
Also the business of sending slaves from Virginia and Maryland (principally) “down south” to staff cotton plantations was mostly carried by coastal shipping, much of which was financed by the North.