No. It is one of essential quality, because your employees are working for you by mutual assent and your slaves aren't. Employees can quit and work for someone else if they decide it's in their interest, whereas slaves are stuck with you no matter how you treat them.
By your logic, the employee is only ever free when possessed of the freedom to serve some other, instead, at their own discretion; insofar as doubt, a paucity of jobs, the risk of losing health coverage and the like might erode their confidence in this ability, you would regard them as being in chains. Thus it must be that it is with the risks presented by the unknown, that they find their interest lies; elsewise, none of them would ever be free without a job offer in hand.
That's not a difference in degree at all. It's a difference in kind, and the distinction isn't even close.
I feel that you have let me down somewhat with your previous explanation of the difference between the employee and the slave, because it is clear then that you do not at all abide by the difference you set out. It is not, after all, the employee's ability to pursue his own self-interest that makes him free rather than a slave.
Actually, yes it is. I'm sorry you feel let down, but your side of this discussion is frankly indefensible.
I think that when you introduce the supplemental notion of the qualitative difference, you take some steps back from a stock libertarian talking point that you might or might not have intended to support. At least, a proper response would require some elaboration of the notion of employee liberty.
I can sympathize if you don't have time or energy for that, and I'm happy to let the matter rest inconclusively. For what its worth, I'd actually be rather interested in whatever ideas you developed to articulate your position, as they would be a more useful reference point than I currently possess the next time I encounter someone making a reference to employee liberty I feel deserves to be tested.
I'm pretty sure I had to read about a few slave revolts for history class when I was a kid. And the underground railroad.
It really is just a difference of degree. But there's that saying, sufficient quantity has a quality all it's own.