In fact, plantation owners used their money to buy goods from the North and elsewhere or they invested the money in New York banks. They got full value for the money they earned from exports. Northerners then could use the money they earned in those exchanges to buy foreign goods.
This was (as we all should know by now) the subject of a bitter polemic between Thomas Prentice Kettell (Southern Wealth and Northern Profits) and Stephen Colwell (The Five Cotton States and New York). Colwell demonstrated the fallacies of Kettell's thesis.
In fact, plantation owners used their money to buy goods from the North and elsewhere or they invested the money in New York banks. They got full value for the money they earned from exports. Northerners then could use the money they earned in those exchanges to buy foreign goods.
Let us say that everything you just said is absolutely true.
What happens to the economic conditions in the North when 72% of their import money gets cut off?