On the OP’s question, there’s actually quite a few scenarios we should be considering, not just one where, in Libertopia, a person comes across him and makes this deal. What if, he comes across him and chooses to do nothing? What we’re in an existent nation and the person chooses to do nothing? What if the man was originally planning not to help him, but the dying man offers him all he owns if he helps him?
My point is, there’s no accounting for morality, particularly in this sort of situation. No matter what government type we have, we have to hope that most people are generally moral and will help. If we have no law to force someone to help, then sure, some people might help, but if we legally require them to help, he may do so at incredible cost to himself, possibly including his death as well. So, I think moral arguments against libertarianism in this sort of question is missing the point, people should still be expected to be moral, even if they’re not legally required to do so.
And as far as the sort of deal goes itself, there’s a few different ways it might pan out. It’s quite possible that such a contract could be found coercive, but if neither party wants to dispute it, it shouldn’t matter. Obviously of lesser consequence, but I’ve been on the short end of deals before and not particularly cared for absolute equity. So, it’s possible that the dying person takes the deal, is rescued, and despite that many might think he was coerced, might be okay with the result. If he is legitimately okay with it, I see no reason to force the law into it.
That said, if he’s not happy with the deal, this is where you’re going to see quite a bit of divergence in libertarian thought. Some libertarians might consider that a coerced deal, taking advantage of the guy, some might think it’s perfectly fine, and some might find somewhere in the middle. One of the biggest misconceptions out there is that libertarians have a monolithic way of thinking and that that way of thinking is essentially equivalent to anarchy.
In my personal view, this is similar, but not equivalent, to price gouging. I don’t particularly have an issue with higher prices in the wake of an emergency, higher demand and lower supply means higher prices, yes, there is some moral obligation to help and not charge ridiculous prices but, again, we run into the moral problem I mentioned earlier where we either legalize it and risk people getting ripped off, or make it illegal, and risk people not getting supplies they could use. But that’s neither here nor there.
This situation is different because it’s a single event. The person who is saving him isn’t making any sort of investment in bringing in fuel or generators in an emergency, he is just in the right place at the right time to see the person dying. The thing is, I think it clearly fits coercion, even if the person offering the deal isn’t the one doing it. My reasoning is, any reasonable person, being aware that wealth is worthless if they’re not alive, would basically have to accept any deal he were offered because, being out in the desert, he can reasonably expect that if that person doesn’t save him, he’ll die. So it is an implicit bargaining against his death.
That said, it doesn’t mean that the cost is flat either. If the guy is out in a jeep and all he has to do is throw him in the back seat, it’s a lot different than if he’s on a camel, has to abandon his cargo to put him on the camel, and then double back.
Anyway,I think most libertarians, and people in general, would agree that the deal in the OP would be considered coercive, or at least made under duress. I also think that most libertarians would agree that part of the governments job isn’t just to enforce contracts, but to ensure that, if there’s a dispute about them, that they were made without coercion or under duress. As such, depending on the specifics, if disputed, I think that a civil trial would find that the contract was entered under duress, and he probably owes the man some sort of reasonable payment, but not necessarily everything he owns. As for how much, that would depend upon the findings of the trial.