As I understand, the original is not clear on whether:
1) Any one who seeks to use his blood, must first ask his permission.
2) Donating his blood automatically makes it available to whoever needs it.
I think your rewording still leaves that ambiguity, somewhat (though it does seem better), because i keep reading that as:
> he must allow it to be used..
Perhaps it would have been best to right it to break it down into two sentences.
He can donate blood for himself.
If someone needs it, they have to ask for his permission.
or
He can donate blood for himself,
but then this makes it accessible to whoever is in need of it.
He has to permit anyone - in other words they're not really asking him permission. I read that sentence to mean that he doesn't actually have any say over who gets to use his blood.
Is there another source for this? Using phrases "has to" and "anyone" implies requirement.
In fact, the very next sentence is "This leaves Thomas dependent on other Rhnull donors", which re-enforces my reading of the previous line. If he could have a private stash, he would not be dependent on other donors.
Standing alone it's impossible to derive the true meaning; it's the perfect doppelganger.
Based on the subsequent sentence I decided the author was in fact claiming that he couldn't save blood for his own exclusive use, but then immediately thought that can't possibly be true.
Poor machines will never learn to speak this language!
Fairly sure he's reading it correctly - the article goes on to talk about how he has to depend on other donors, and how he has to be very careful since there aren't any nearby.
The sentence by itself is unclear, but the context of the surrounding paragraph makes it pretty clear that while he can donate blood to have on reserve in case he needs it, he might be screwed if someone else needed it, because he has to allow them to use it.
That sentence could have been worded better. This is the way I read it: