As pointed out by David Gordon in his book The Essential Rothbard, Rothbard objected to the view that a contract could be derived that allowed one to voluntarily enslave themselves to another individual given the fact that the will is inseparable from the body and thus not transferable. For understanding purposes we may say that the body was first homesteaded by the will and cannot, yet, be alienated.
“Unfortunately, many libertarians, devoted to the right to make contracts, hold the contract itself to be an absolute, and therefore maintain that any voluntary contract whatever must be legally enforceable in the free society. Their error is a failure to realize that the right to contract is strictly derivable from the right of private property, and therefore that the only enforceable contracts (i.e., those backed by the sanction of legal coercion) should be those where the failure of one party to abide by the contract implies the theft of property from the other party.” Ethics of Liberty page 133
The problem with this theory is that it doesn’t take into consideration that, although the will may be inalienable, it doesn’t mean that a contract can’t be derived in order to enforce servitude. Take for example an individual that voluntarily enters into a contract that states, at some point in time, if they relinquish their servitude, they may be viewed as a trespasser and subject to whatever penalty predetermined. Now, in this contract the will was never transferred, since such a transfer is currently scientifically impossible, but the right to trespass was contracted, conditional upon the fact that servitude is delivered. If, for some reason, such servitude is relinquished than the slave becomes a trespasser subject to whatever penalty advertised. Therefore, although the will cannot be contracted away, it is still within reason that a contract may be devised under such a theme as “trespassers will be shot” in order to maintain voluntary slavery.