What is this?
The Mishna (Bava Metzia 44a) says that although paying money for an object does not effectuate a kinyan, there is still some obligation to abide by the sales agreement. This is in the form of a semi-curse known as a Mi Shepara (“He who pays back from the generation of the Flood and from the people of Sodom). Besides a moral obligation that Mi Shepara engenders, there are also legal ramifications. The Shulchan Aruch (Choshen Mishpat 198:15) says that until the one who backs out accepts the Mi Shepara, the sale is still intact and so if the money given is lost, it is the responsibility of the seller to reimburse for it.
The source:
Bava Metzia 47b-48a
R' Yochanan said: Biblically, money acquires [movable items]. Why did they say that only moving the item acquires it? Lest the seller say [after receiving payment], "Your wheat was burnt up in the silo."...
Reish Lakish said: The requirement of moving the item is biblical! The Torah says, "When you sell merchandise to your friend, or you buy from the hand of your friend" – that which is passed from hand to hand...
[The Talmud challenges Reish Lakish:] A Mishnah taught, "They have said, 'The One who punished the generation of the flood will punish one who does not stand by his word.'" If transferring money effects a transaction, that's why he is under this curse, but if transferring money does not affect a transaction, why is he under this curse? Because of words.
But are words sufficient to trigger this curse? Didn't we learn, "R' Shimon said: They have said that transferring a garment acquires a gold dinar but transferring a gold dinar doesn't acquire a garment. However, they have also said, 'The One who punished the generation of the flood, the generation of the dispersion, the citizens of Sdom and Amorah, and Egypt at the sea, will also punish one who does not stand by his word.' Verbal commerce does not acquire, but the sages are not pleased with one who reneges." And regarding this, Rava said that [in a case of mere words] we only say that the sages are not pleased with such a person!
True, but where there are words and money, then he is under this curse...
