Slaves WERE money stupid, at the time $4 billion dollars.
Which is exactly why the Northern powers wanted them to keep working to stream money into their pockets. The South leaving takes that labor force out of the control of the Northern power brokers.
The South did 500 million in trade with the North, and 200 million in trade with Europe.
That was almost all slave produced money.
The corrupt evil Northerners who enriched themselves off of slave money wanted to keep that gravy train going.