Kamara's form for SKC had been stellar since early May with seven goals in ten starts prompting Vermes to admit it was a hard call to sell.
"Obviously, it's never an easy decision when you're making the choice to sell a player," said Vermes.
"At the same time, it's part of the international soccer business. It wasn't an easy one, but at the same time I think it was a decision between both parties that was the likely choices.
"It's not like we've been fielding calls like crazy. There was another club that was interested, but it wasn't to the point where we felt comfortable doing a deal for many different reasons. It wasn't just the number.
"Then this one came up, and I don't want to say really quick, but a little quicker than normal - just over the last four or five days here."
Middlesbrough currently sit 15th on England's tier two table with six points from five matches, having scored six goals.