NUM355 The sixth category [of liars] -- one who promises to help another, and [subsequently] gives the lie to his words by reneging on them. After he said he would help his fellow man with a promise, and after the other has placed his trust in him, he must not violate his promise--this is the path followed by those who lie, and is comparable to a man who violated his covenant, as the pasuk says (Tzefanyah 3:13), "The remnant of Yisrael shall do no injustice, nor speak falsehood, nor shall a deceitful tongue be found in their mouth." [This verse is brought as proof in Pesachim (91a) that the Jewish people keep their promises.] The same applies to one who told another that he would give him a small gift, even if he did not mention a promise. Our Sages said (Bava Metzia 49a) that reneging on this is a breach of trust, for the other relies on him and trusts him to surely give him a gift, since it is [only] a small one. Moreover, if the intended recipient is poor, even if it is a large gift [i.e., and as such the recipient does not really expect to receive it.], recanting on such a commitment is a very severe offense, for he has taken [the equivalent of] an oath, and the pasuk says [this verse], "He must not violate his word." ["Any utterance of your lips you shall observe and carry out… Whatever you spoke with your mouth" (Devarim 23:24). The Talmud derives from this that when one offers a gift to a poor man (even without a promise), it is considered as if he has made a charitable vow (Rosh Hashanah 6a.)]