"Joseph was rightfully no more a Slave to his Brethren, then they were to him: and they had no more Authority to Sell him, than they had to Slay him. And if they had nothing to do to Sell him; the Ishmaelites bargaining with them, and paying down Twenty pieces of Silver, could not make a Title. Neither could Potiphar have any better Interest in him than the Ishmaelites had. Gen. 37. 20, 27, 28. For he that shall in this case plead Alteration of Property, seems to have forfeited a great part of his own claim to Humanity. There is no proportion between Twenty Pieces of Silver, and LIBERTY." - Samuel Sewall, "The Selling of Joseph" (1700)
Sewall was one of the Puritan judges who took part in the infamous Salem witch trials and was the only judge involved who later recanted his participation. He was friendly towards Indians, apparently thinking them a lost tribe of Israel; and in the essay "The Selling of Joseph" he became one of the first colonists to lay out what would become standard abolitionist arguments against slavery.
For more on Sewall, see the fascinating entry on him in Head and Heart by Gary Wills.