If someone signed a contract obligating them to an indefinite period of indentured servitude "in the extremely unlikely event that a debt would be unable to be paid off", the NAP says that's fucking legit!
Now don't get me started on the hypocrisy of disrespecting intellectual property, while simultaneously worshipping the sanctity of contractual agreements.
Slavery was codified in law. If you stop "slaving" by, say, running away, or help a runaway slave, you are breaking the law. We all know what happens when you break the law: men with guns come chase you, and then either imprison or kill you. In this case, they take you back to your owner.
Your indentured servitude example is a contract. If you break that contract, you have only broke a contract between you and your "owner," at which point the two of you would have to go to an arbitrator, mediator, or civil court to figure out how to resolve the breach of contract. If I was mediating or a member of a jury, my "ruling" would have been that the indentured servant and the "owner" should figure out some other ways to resolve their debt issue, and that the indentured servant should be deemed as untrustworthy for entering into indentured servitude contracts, and thus not be allowed to enter into them any more. At least until they rebuild their trust. This indentured servant would have to start from scratch, maybe only doing short one or two days long indentured servitude contracts at most, and would take a while to build up their reputation enough to become a professional long-term indentured servant again.