Good point, but this may fall in with my "other factors" point.
The capital should benefit all who are involved, not just the boss. If the first person had hired the second for $80 a chair and, after a while, bought a machine with the exploited profits (from the second person) to allow the second person to make 10 chairs in the time it normally took to make one, who's money did they really use? The exploited money. Furthermore, in society now, the agreement probably wouldn't have been for $80 a chair, but $80 a day (and it takes 1 day to initially make a chair. There would probably be a quota or something). In today's world, would the second person now get $800 a day with the boss getting an extra $200 a day for doing nothing except supplying the new machine? Or would the second person get a small raise (to $100 a day) to make it seem like they get something, and the boss gets $980 extra a day?
It's most likely the second way. But either way, it all started with somebody being exploited for the boss to have enough capital to expand the business. The boss did take the risk to invest more in the production of chairs so he may be entitled to a bigger chunk of the profits, but the money he used to do that was exploited, initially.