Arguably, their contributions helped significantly to boost Germany's war economy. But they did so involuntarily , not as willing allies, but as reluctant, quasi-prisoners and enemies of Nazi Germany. Their sufferings deprivation of any legal protection resembled that of slaves. Their powerlessness in the face of their cruel and murderous German masters indeed made them feel like slaves. But slaves they were not, not according to a strict definition of slavery.
Slaves are, firstly, an economic commodity, a property and an investment.
Slavery is both an economic transaction between seller and buyer, but also a legal arrangement. The slave is property as defined by law or custom, a condition, for example, that empowered the owner and, usually, consigned the slaves' progeny to inherited slave status. None of this was operative in the German use of their involuntary foreign labor. Nevertheless, the foreign workers in Germany had not been purchased. Nor were they subject to being sold by one factory to another.