I don't recall seeing anything that looked like contrary evidence to me.
We have the financial records for this time period. Perhaps you haven't seen them?
Now if you will take those figures from both the North and the South as 100%, then you will find the South produced 72% of the total, while the North produced 28% of the total.
So how was the North getting it's hands on more than 28% of the European money?
The North had 4-5 times the population, yet were only paying 1/4 of the total tax burden?
How does that work?
Probably a factor. Probably not the dominant factor.
700 million dollars per year in a 4 billion dollar per year GDP economy is a pretty serious factor.
The South produced 200 million per year in Trade value with Europe, and 500 million per year in Trade value with the North.
There was 700 million dollars per year at risk for the Northern power structure if the South got away from them.
But no, they only cared about the poor slaves whom their representatives voted to keep in slavery forever. (The Corwin Amendment.)
Here are some records which show a larger picture. It is not so simple as merely Southern exports of goods vs Northern exports of goods. Add in specie (gold and silver), which has to count, the balance for 1859 is much closer:
South 198 (rounded off) millions Non-South (rounded off) 135 millions.
Now add in contributions from immigrants (who brought money from overseas to the USA), which counts against exports, which was at least a few million dollars, mostly in non-south areas, and earning from services rendered overseas by US companies (most shipping) another few million (mostly from the North) and you are coming up with pretty near parity in non-South and South contributions to the balance of payments.
There are more complexities, of course. Many of the immigrants sent money back to Europe; money was invested (mostly by the British) in American companies, particularly railroads, mostly in the North. Loans were made from abroad to people in the US. Those may have been more in the South.
The balance of payments was not perfect, but bounced up and down from year to year. 1859 was a pretty high year for Southern exports, but also for export of gold and silver.
It is obvious the South was not being financially destroyed by the North. Some in the South may have thought so.
A serious problem for the South was the industrial revolution was undercutting the economic viability of slavery. A lot of people at the time could see a slave based economy would not be viable in the future. That undercut the entire slave owning culture of the South.