There is a revenue stream that was the economic life blood of states like Virginia where staple crop plantations were no longer economically viable. Either overtly or not, the true cash crop was human beings. It was the “extras” born on those plantations that filled the voracious need for replacements & growth in the Deep South. It was an accepted management dictum that slaves only had a seven year useful working life. The pestilential rice producing areas probably did not achieve even that. The steady flow of extras from the surplus births in the border states is what made the slave/cotton connection possible.The problem with slaves as a source of wealth was that that form of wealth was not recognized in New York, London, Paris or Frankfurt.
The second problem was that it was mobile, it wasn't real estate or a factory. Slaves were more like automobiles, and could be difficult to locate when the time came to repossess.
After March 2-3, 1859 it became obvious that making good on slaves as security meant auctioning human beings.
https://www.theatlantic.com/business/archive/2014/07/the-weeping-time/374159/Slaves were not a merchant vessel, a revenue producing RR right of way, an iron factory, or recognized paper security. They had no value except if the economic system of the 7 cotton producing states was viable, economically and politically. Anything that created doubt about that viability decreased the value of slaves.
Evidence of that was the large number of Chinese that were brought in to replace the children born in border states after the Civil War.