But then we get to the last statement. "Refuse, and things will not seem so well." There are (at least) two ways I can think of to view this. One possibility is that Sauron is not actually proposing unilateral contracts at all. After all, a reasonable interpretation of his offers would be that they were unilateral, but we're talking about Dark Lord Sauron who really wants to enslave all the free peoples. He might not contemplate reasonable contracts. In fact, given the ease with which agents of the Dark exact damages from lackeys who fail them, it seems possible that in Morder, where the shadows lie, all contracts are bilateral, no matter how ridiculous it seems to contemplate such a thing. So maybe what he's saying is that if they fail to produce the ring or any information, he'll exact expectation damages.Link (via MeFi)
But this reading doesn't really make sense given the express language of the offer. The Messenger from Mordor isn't claiming that if they fail to deliver the ring they'll suffer expectation damages unto the fourth generation. He's saying "Refuse, and things will not seem so well." The "Refuse" comment modifies the offer. The law doesn't contemplate expectation damages if you don't accept an offer, although Sauron might.