What level of personal responsibility is there for a person to keep a verbal agreement?
Our Gemara tells of Rav Kahana, who received partial payment for an order of linen. Before the full order was delivered, the price of linen went up, and Rav Kahana – who did not want to lose money on the transaction – asked Rav what his obligation was in filling the entire order. Rav responded that he was only obligated to supply the linen that had been paid for; the rest of the order was a verbal agreement, and backing out of it was not a problem. The Gemara follows this by pointing out that although this is Rav’s position regarding verbal agreements, Rabbi Yohanan ruled that verbal agreements are binding and must be kept.
Questioning Rav’s position, the Gemara quotes a baraita where Rabbi Yosei son of Rabbi Yehuda expounds the passage in Sefer Vayikra (19:36) that discusses keeping honest weights, teaching that a person’s hen or lav – his statement “yes” or “no” – is sacrosanct and must be kept. In response, Abaye says that Rabbi Yosei’s teaching refers specifically to a situation where a person is ehad ba-peh ve-ehad ba-lev.
The expression ehad ba-peh ve-ehad ba-lev (literally “one in the mouth and one in the heart”) is understood by Rashi to mean that when a person is negotiating a business deal he must be honest in his statements so that piv ve-lebo shavim – what he says should reflect his true intentions, and he should not make a promise that he does not intend to keep. Thus, in our case, since Rav Kahana made his commitment in good faith at the time of the original negotiations, Rabbi Yosei’s teaching would not apply to him. The Rambam understands Abaye’s explanation as meaning that Rabbi Yosei’s teaching is not referring to business specifically, rather that a person should not flatter others or say things when he does not truly believe them.