The took their profits in dollars.
The foreign money was in New York banks and available to buy imports.
Or do you really think that the planters had both all the foreign currency they "earned" and all the dollars they exchanged the foreign money for?
Or that they had some moral right to all the foreign money they'd exchanged and the US money they exchanged it for?
And who made the exchange? And why does it matter what form the European payment for their products was in? It was still their European money that must be used to buy the European products imported into the Nation.
The foreign money was in New York banks and available to buy imports.
Of course it was, and who would be loath for this economic activity to move elsewhere?
You are trying to play some "hocus pocus" by invoking middlemen to cloud the origin and ultimate disposition or the money, but when you summarize the whole thing, European imports were being paid for by Southern produced wealth.
The products bought by Northerners from Europe were bought using money they had gotten from Southerners through intrastate trade that was deliberately pumped up by Washington's Protectionist policies.