I am not doing much in depth learning nowadays, but just occurred to me at the beach to ask an obvious question. With a gift we say that if the external circumstances show it was given with a mistaken assumption, then the gift is invalid--even though it was signed and sealed in a legitimate court of law according to halacha. To Tosphot [Ketuboth 37] this applies also to sales. But a forced sale is valid. You might answer that in a forced sale, there is no mistake about the circumstances. But even in the first case, it seems that the mistake in circumstance also ''forced'' the sale.

What I mean here you can see in a few examples. Let's say a person has heard that his son died in a faraway country, and then signs away all his property. Then we discover that his son is alive. We say that gift is invalid. 

But if someone ties up a person, and forces him to sign a document of a sale, that sale is valid- since we say because of the circumstances, he really did intend for the sale to be complete and valid.