someone who works for free is also a volunteer.
An office worker might not be a volunteer,
If work without pay is volunteer, and with pay is slavery, then we are in a place I've never been. Or heard of for that matter.
If you replace "pay" with "coersion" it makes more sense. To own slaves, one needs to compensate them with food and housing, or some kind of "pay" that keeps them alive and one's slave. Pay has evolved in the time since serfs. To call modern wage slaves simply 'slaves' may be a bit of an overstatement, but it still rings true enough to get my point across.
Anyone familiar with sharecroppers?