I cannot find any significant difference between labor and slave.
Once I have tried to figure out how the history books explain the problem of the slavery. When I was in Korean, I went to book shop and I skimmed about 10 books. They usually uses the concept of The Human Rights. They said the problem of the slavery system is the master exploits their slaves too much. But it is hard to explain how much is too much, so that they use the concept of the Human Rights; For example, the problem of the slavery system is the slaves cannot have the Human Rights.
Considering the fact that the slavery is abolished after the American civil war in 1861, the concept of the Human Rights has been established after the slavery has already disappeared, 1948. It is hard to say when the concept of the Human Rights had fixed. I think at least it cannot be before 1789 when the French revolution occurred. I believe that although somebody can find any evidence that there was mere current for the abolishment before the slavery disappeared, it would not be hard to find any evidence that the concept of the Human Rights is quite new.
My point is that actually using the Human Rights to explain the problem of the slavery system does not make sense. If we can criticize any society using a concept which will be developed in future, we can also criticize capitalism using any concept which we don't know yet.
In brief, what is the problem of the slavery system? We can easily find the wage slavery or the contract slavery is almost similar with the wage labor. I believe that if somebody who thinks the slavery is bad thinks the capitalism is good, his thinking is inconsistent.