Exodus:
21:2 If thou buy an Hebrew servant, six years he shall serve: and in the seventh he shall go out free for nothing.
21:3 If he came in by himself, he shall go out by himself: if he were married, then his wife shall go out with him.
21:4 If his master have given him a wife, and she have born him sons or daughters; the wife and her children shall be her master's, and he shall go out by himself.
21:5 And if the servant shall plainly say, I love my master, my wife, and my children; I will not go out free:
21:6 Then his master shall bring him unto the judges; he shall also bring him to the door, or unto the door post; and his master shall bore his ear through with an aul; and he shall serve him for ever.
21:7 And if a man sell his daughter to be a maidservant, she shall not go out as the menservants do.
You get to "buy" these "hired" workers. They have to serve you for six years. You have to let them go after seven years -- unless they want to stay with the wife you've bought for them, in which case they have to serve you for the rest of their lives.
Doesn't seem to be an option where female "hired hands" ever get to leave.
Oh, but none of that is slavery, no. Couldn't be. Nope.