An interesting question. We do know there were white slaves later on, when people were legally white if they were 3/4 or 7/8 white, depending on the state they lived in. So long as your mother was a slave, you were a slave, whether you were legally white or not.

The basic info we have about Casor is this: "In 1653 John Casor, an African employed by Johnson, filed what later became known as a freedom suit. He said that he had been imported as a "seaven or eight yeares" indentured servant and that, after attempting to reclaim his indenture, he had been told by Johnson that he didn't have one. According to the civil court documents, Casor demanded his freedom."

What that appears to mean is that Casor had been sold by Africans to slavers and sold to someone in the colonies at a time when slaves were treated like identured slaves and freed. That had been Johnson's experience. But Johnson argued slaves were different, and the court agreed.

Johnson might've had white indentured servants who, at the time, would've been treated the same as slaves.