Well technically that would make little sense to the Sangui, he'd gain nothing out it all either. 
But perhaps you could have it give you what you ask for, not what you need most. So when he'd be done drinking and leaves you near dead, he would give you water, which you no longer really need all that much.

But I have one sidenote, this Sangui belongs to what I would categorize as mid to high fantasy, while our current setting is more of a low fantasy one. In fact, it is mostly like how one would imagine an alternative universe, which has the same basic rules and laws, but just turned out different. So if you are thinking of something, just imagine something that could exist on our world, but it just happens that it doesn't (or it did, but is extinct now). You can add a few fantasy aspects to it for sure, but try not to overdo it.

Also, I said a while earlier (but I forgive you for not reading through it all) that I would prefer it if no sentient beings were to be found, so no humanoid characters if you can avoid it.

That said, you wont have to change your posts so far, but it is just something to keep in mind for another time.