There is a difference between voluntary slavery and involuntary slavery. In Jewish culture slaves voluntarily served others even if there masters were oppressing them. Hence why there are laws in the Bible that protect slaves.

Exodus 21:16 "He who kidnaps a man, whether he sells him or he is found in his possession, shall surely be put to death.

Clearly this is referring to taking people into slavery against their will.

Nowhere in the scripture will you find involuntary/forced slavery condoned. But you will find laws that protect slaves who voluntarily sold themselves and also after a certain amount of time slaves were to be given the choice to be free or remain serving their master. Think of indentured servitude.