It comes down to the difference between that which is alienable and that which is inalienable.

To say something is 'alienable' means you can transfer it to another person, while the word 'inalienable' describes that which cannot be transferred.

A man's property and the resources he owns are alienable. He can enter into legally binding contracts that involve the transfer of such things.

But a man's will is inalienable from his self. He cannot separate the two, which means he cannot enter into a legally binding contract that involves his self.

Such a contract would violate the right of his future will to ownership of his future self, making it contradictory and legally bogus.

Hence, if a person enters a contract of voluntary enslavement, that can only be considered as a promise, not a legally binding transfer of an ownership title.

Should a person who entered into an enslavement contract decide to stop being a slave at some point in the future, the contract then becomes void. The slaver cannot legally enforce this contract in a court of law.

If the slaver attempts to physically restrain him from leaving, he becomes an aggressor who shall be prosecuted.

P.S. Note that this makes the 'social contract' of citizenship under a government completely bogus. It's even worse in this case as it involves the will of someone else in the past violating the right to self-ownership of other people in the future.

Every modern government is essentially a slaver if this reasoning is taken to its logical conclusion.