“How much is the discount?” asked Mr. Miller.
“In a third case, someone sent engagement gifts to his kallah, but she retracted,” replied Rabbi Dayan. “She pays for what ate as dmai basar b’zol, which the Gemara [B.B. 146b] clarifies as a one-third discount. Rashbam [s.v. v’shamin)] extends this to any case that one pays for eating without expecting to pay for it, as in the two previous cases.” (E.H. 50:3)
“Why should we evaluate this way?” asked Mr. Feldman.
“A person who benefits at another’s expense has to pay for the benefit,” explained Rabbi Dayan. “The benefit, though, is not necessarily the cost of the item, because he could have sufficed with something cheaper. For example, a simpler cake would have sufficed here. Similarly, the heirs or kallah might not want to pay for meat, and could have eaten bread. Nonetheless, we presume that someone would be willing to buy something better at a one-third discount.” (See also Aruch Hashulchan C.M. 363:21.)
“Thus,” concluded Rabbi Dayan, “you should pay for the fancy cake 400 dollars, two-thirds of its price.”