People enter into slave contracts all the time in the BDSM scene.
However, these contracts are legally unenforceable.
The interesting thing about legalized slavery is that it requires the state to enforce it. It requires the state to treat a slave differently than a free person.
What happens when the master tells the slave to go out and pick the cotton, and the slave refuses and the master beats the slave? Has the master committed assault and battery? Or does the law recognize that a master may beat his property in a way that would be assault if he did it to some guy who just happened to be staying at his house? Can a slave bring a lawsuit, or enter into contracts, or vote, or serve on a jury, or own property, or testify in court? Roman law used to allow the testimony of slaves, but only if the testimony was extracted via torture.
If we’re talking about some future Libertopia, where people are free to enter into any contracts they wish, the government has no reason to treat a person who sold themselves into slavery differently than any other person. And therefore, many of the classic features of slavery are impossible.
Under current law, it is very very difficult to get courts to enforce agreements for specific performance. Suppose I contract to whitewash your fence in return for $10. You give me the money, but I don’t whitewash your fence. So you sue me. The court won’t order me to whitewash your fence, rather they’ll order me to return the $10. And the reason why is obvious–if I refuse to obey the court order to get out there and whitewash the fence, what are they going to do? Hold me in contempt of court, I guess. Whereas if the court orders me to pay the $10, and I refuse, they can go to my bank and order the bank to take $10 out of my account and hand it over to you, or go to my employer, or put a lien on my house. There are all kinds of ways to extract money out of an unwilling person, but very few ways to get them to provide specific performance.
So, if you “sold yourself into slavery”, which would mean you agree to sell your entire future labor to someone in return for some lump sum, what happens when you don’t work? Your “master” can go to court and sue you and ask them to enforce the contract. But that means the court will order you to repay the purchase price, they won’t order you to get to work. And if the “slave” doesn’t have the money anymore? The “master” is free to try to collect, but how do you collect against a broke person? If bankruptcy is possible, the slave can pretty easily discharge the debt, so I guess we’d have to eliminate bankruptcy and bring back debtor’s prisons for this to work, then the slave would have a choice between working for the master or a government workhouse.
It’s pretty easy to see that if slavery is merely a private contractual agreement between two parties that the arrangement isn’t going to resemble classical slavery very much. For that you need either state enforcement of slavery, or a state so weak that a powerful person can force you to work at gunpoint for years without fear of being charged with assault.
People have been privately held in slavery in this country in that fashion–some guy kidnaps you, takes you to his secluded house, locks you in chains, and beats you if you don’t do what he says. Except, if the cops find out, that guy is going to prison for a long time. That doesn’t count as legal slavery.