If they subjected him to
distress until he finally relented and sold to them what they demanded,
his sale is a valid sale, and he cannot retract.
What is the reason?
* Is it because any sale is forced, since the seller needs money? - No, it could be different when it is against his will.
* Is it because a recalcitrant husband who is forced to give a Get is indeed divorced? - This is not a valid proof, because it is a mitzvah to heed the directive of the Sages.
Rather, it is derived from the following ruling: if they subjected a woman
to distress until she agreed to be married, the marriage is valid. This
proof stands. However, the Sages later annulled such a marriage: he
behaved improperly, therefore, they behaved improperly with him.