Why did Solomon Northup's captors not keep him in New York?

New York was a free state, thus, Soloman's captors, who he believed to be his employers, needed to bring him to a slave state in order to place him in bondage and connect with a slave dealer.

There must have been some misapprehension—some unfortunate mistake. It could not be that a free citizen of New-York, who had wronged no man, nor violated any law, should be dealt with thus inhumanly.