No it isn't. It's the consequence of goods being shipped into the United States.
Which are only shipped into the United States as a result of the exchange value for goods shipped out of the United States.
If the South wasn't producing 200 million dollars worth of export value, (per year) Europe wouldn't have been shipping in 200 million dollars worth of import products. (per year)
This isn't rocket science. Trade must more or less balance over a given period of time or you will have what is called a "Trade Deficit" or "Trade Surplus."
The steady state solution for this equation is 200 million in import value is the result of 200 million in export value.
You are resisting this because you do not like what it proves. It proves that in 1860 the vast majority of the US government Budget, and a huge amount of the Northern Economy was heavily dependent upon slaves.
It proves that when the South became an independent economic system, it caused a severe loss of money to the North Eastern economy.