My weekly halachah column:
In parashas Kedoshim (19:36), the Torah commands: “Just balances, just weights, a just ephah, and a just hin, shall ye have”. The plain meaning of hin is a particular measure of volume, but the Talmud (Bava Metzia 49) interprets the word hermeneutically in the sense of hen (“yes”), and thereby derives the exhortation that “your ‘yes’ shall be just, and your ‘no’ shall be just”, which it further explicates to mean that one should honor one’s commitment (even when not formalized by a contract or any other civil ritual), or at least that one should not make a promise in bad faith.
The halachah is that both these types of faithlessness are unacceptable. Some authorities apparently understand that honoring one’s commitment is a bona fide Biblical commandment, and the Minchas Chinuch (#259) assumes that according to this view, a Beis Din will actually compel one to keep his word. The Minchas Pitim (204:11, and see 183:4), however, rejects out of hand the possibility that a mere promise is be enforceable by Beis Din. It is generally agreed, however, that one who fails to keep his word is considered a sinner, and may be publicly declared to be such (Shut. Maharam b. Baruch [Prague] #949, Shut. Maharam Mintz #101).
There is a major unresolved dispute over whether the obligation to keep one’s word applies even when circumstances have changed significantly from the time of the promise (see Rema Choshen Mishpat 204:11, Minchas Pitim Shiarei Minchah end of 204:11). There is also some debate over whether the obligation applies to a commitment made not in the presence of the beneficiary, or to a minor (see Minchas Pitim 204:8), or in various situations where a formal, contractual agreement would not be binding, such as a commitment regarding property that one does not yet own (davar she’lo ba le’olam or she’aino be’reshuso – Shut. Pri Yitzchak 49-50) or conditional or penalty obligations (asmachta – Erech Shai Choshen Mishpat 14:5).
My weekly lectures, on the same topic: Y version, K version.
[I generally deliver two versions of the same basic lecture. The Y version takes place first, and is less formal and delivered in a noisier environment than the K version. The K version includes handouts, usually benefits from additional preparation, and sometimes has some of the rougher edges of the Y version ironed out.]