62:16 It is so important for a person to keep his word that, even if no money has changed hands, nor has the object been earmarked for the buyer, even if all they have done is agree on the price, nevertheless neither should renege on the deal. If either the buyer or seller does back out of the deal, he is considered untrustworthy and the Sages express their disapproval of him. It is appropriate for a Jew always to keep his word as per Zephaniah 3:13, "The remnant of Israel shall neither do any iniquity nor tell lies." A person who is in proper awe of G-d should keep even what he decided in his heart, so if he thought to sell the item at a certain price and a potential buyer offered him more, the seller should only accept the amount that he had decided upon, as per Psalms 15:2, "he speaks the truth in his heart." Similarly, if the buyer decided that he would pay a certain amount, he should not go back on this. In any dealings that one has with his fellow man, he should fulfill even the decisions he made only in his heart, of which the other party is not aware. For example, if one decided to perform a favor for the other and is able to do so, then he should carry out as he intended. However, he need not carry out intentions that are only for himsef, so long as they do not involve the performance of a mitzvah. This is true even if he stated such intentions out loud.
62:17 If one person told a friend that he would give him some small thing as a gift and the other relied upon him, if the first person changed his mind and didn't give it to him, he is considered as untrustworthy. In the case of a large gift, however, he would not be considered untrustworthy because one does not rely on being given such a thing. In any event, when one tells another that he will give him something, he should have it decided and not change his mind. We are not permitted to say one thing and intend something else, as we see from Leviticus 19:36. There, discussing measures of volume called an ephah and a hin, the Torah tells us, "You shall have a just ephah and a just hin." The Talmud in Baba Metzia 49a points out that the measure "hin" may also be read as "hein," meaning "yes." We should have "a just yes," meaning that one's word should be reliable. All of these gift-giving rules only apply when one has said he would give something to a person of means. When it comes to a person in need, whether one said he would give him a small thing or a large thing, one may not retract because it is like a vow. Even if he only decided in his heart to give a poor person a gift, he must fulfill the intention.