The Curse of Mi Shepara
The Talmud teaches that one who reneges on his word, whether in business or in any other commitment, is worthy of the curse of "mi shepara." The mi shepara curse is proclaimed as follows, “He who punished (“mi shepara”) the generation of the Flood, the Tower of Babel, S'dom and Amora, and the Egyptians by the sea will punish one who does not keep his word.” The mi shepara curse is considered to be especially serious and even drastic. Nevertheless, a Beit Din would not hesitate to pronounce the curse upon someone who unlawfully reneges on his word. In order to properly understand the significance of this curse, some background information is in order.
According to Torah law, one legally acquires movable property once one has paid for it. Our sages, however, decreed that in addition to paying for something, one must also perform a "kinyan" in order to be considered the rightful owner of the item. A kinyan is an act that demonstrates acquisition and ownership. It can be accomplished by pulling the item (known as "meshicha") or lifting it (known as "hagba'ah"), among other types of kinyanim. Therefore, according to halacha, even if one paid for an item, one does not legally acquire it until a kinyan is made. The only exception is real estate which does not require a kinyan and ownership is transferred upon payment.
Nevertheless, even though one does not officially own an item until a kinyan is made, one may not renege on one's commitment to purchase it once all, or even part, of the money has been paid. This is true even if one is willing to forgo one's deposit! In fact, one should not renege on a deal once an agreement has been reached even if no money changed hands.The only exception to this rule is if new information, developments, or financial considerations suddenly arise that would have deterred one from getting involved in the agreement in the first place. A seller is also not permitted to renege on his side of the deal once money has changed hands.
One who paid for something and then decides to back out of the deal must retrieve the money from the seller. The seller is exempt from responsibility for the money should he lose it through no fault of his own. If it is the seller who backs out of the deal, then he is fully responsible for the money and is liable if it is lost. One may renege on a deal if the item in question has been destroyed or stolen, and in some cases, if it has or is expected to depreciate considerably.
Anyone who unjustifiably reneges on a deal is not conducting himself as is expected of a Jew, and is labeled by our sages as a "mechusar amana" – a person of untrustworthy character. It seems that the sages enacted the mi shepara curse at about the same time they instituted the requirement of a kinyan in order to finalize the sale of an item. Perhaps this close association between a kinyan and the mi shepara curse is intended to further deter people from backing out of a deal once money has changed hands.
In the event of a dispute between a father and a son in which the father is deserving of the mi shepara curse, the son may not ask the Beit Din to pronounce the curse upon his father due to his obligations of kibbud av va'em. He may, however, ask the Beit Din to remind his father that his conduct warrants the curse of mi shepara. On a related note, children should endeavor to honor any agreements and commitments that their deceased father may have made in his lifetime.
 Bava Metzia 47b-49a; CM 204:4.
 CM 190:1.
 Bava Metzia 44a, 48a, CM 204:1.
 Sma, CM 204:3.
 CM 204:11.
 Chatam Sofer, CM 102; Shevet Halevi 4:206. See also Tzitz Eliezer 8:40.
 Rambam, Hilchot Mechira 7:3.
 CM 204:2.
 Rambam, Hilchot Mechira 7:1.
 Bava Metzia 49a; Rambam, Hilchot Mechira 7:9.
 See Rambam, Hilchot Mechira Chap. 7.
 Noda B'yehuda, YD 69.
 Maharsham 1:40.
 Mishnat Yaavetz, CM 33. See also Achiezer 3:40:3.