The way it seems to have worked in the Louisiana ports is that a ship from Europe would be carrying immigrants who had been unable to pay the fare. When the ship arrived at New Orleans those passengers were held on board until someone agreed to purchase them, thereby paying the ship’s captain for their passage. They then worked for the buyer for a number of years, as a slave. They got no wages as a slave. After their term of slavery was up, they were free to earn a living in the normal way.
An owner could release an indentured servant from his contract early if he became sick, to avoid the expense of caring for him. For permanent black slaves, the slave owner was legally obligated, however, to keep a sick or disabled slave.
Perhaps I am wrong, but my understanding was that they could not do this by sailing into northern ports, because passengers could not be legally sold into slavery from northern ports, even voluntarily.