Economics 101 - Evidence Based Decision Making

Certainly not slavery since they were free to quit their job at anytime.

That precept is why I posted the example of company money. If the company owns the house you live in and you are paid in a currency that is not transferable or portable and you are not someone with a talent that is scarce and needed are you really free to go find another badly paid job?

So as is so often the case the devil is really in the details. The mill workers were certainly not considered slaves in any legal sense (at least in the US) and unlike the case of slaves (again in the US) there was no owner and the company was not free to send hunters after an employee who left and the State Governments were not under a legal obligation to return runaway workers, yet they were every bit as much property as slaves or many sports figures.