Musar su Sofonia 3:13

The sixth section: One who promises his fellow to benefit him, but falsifies his speech and makes his word worthless. Since after he said to benefit him (upon) [with an expression of] a promise, and his fellow’s heart depended on it, he should not profane his promise - as this is the way of falsehood. And it is like a man who broke a covenant, as it is stated (Zephaniah 3:13), “The remnant of Israel shall do no wrong, and speak no falsehood; a deceitful tongue shall not be in their mouths.” And likewise one who says that he will give his fellow a small gift, even though he does not mention any expression of promise. And our Rabbis, may their memory be blessed, said (Bava Metzia 49a) it is because [there is] bad faith with this. As since the gift is small, his fellow counts on it; that he will surely give it. And if it is to a poor person, his evil is great even if the gift was large - for he surely vowed it (since charity is like a vow); and it is stated (Numbers 30:3), “he shall not profane his word.” And likewise one who glorifies himself in public by [announcing that] he will give a gift to someone. And see that he is like one who praises himself about his generosity with this. And this is surely like a promise, so it is not proper that he should go back on his words once he honored himself and boasted about the thing; like the matter that is stated (Proverbs 25:14), “Like clouds and wind - but no rain - is one who boasts of gifts not given.” Its explanation is, just like people are distressed after signs of rain come, but rain does not come; so too is the matter of the man who boasts about a false gift. For that which he boasted about the thing is a sign about the fulfillment of the thing. Hence the man to whom he promised the gift will be distressed when his expectation is disappointed.

But if he does not meticulously examine his deeds very carefully, thorns will sprout instead of wheat. For he will transgress and stumble in the sin of Onaah (fraud) which we were warned against: "And you shall not oppress one another" (Vayikra 25:17) and our sages of blessed memory said: "even to deceive a non-Jew is forbidden" (Chulin 94a). And scripture states: "the remnant of Israel shall not do iniquity nor speak lies, neither shall a deceitful tongue be found in their mouth" (Tzefania 3:13).

But the wisest of men (Shlomo) has taught us that all of this is contrary to the will of the Creator, blessed be He, and the attributes of His pious ones as written: "the righteous man hates a false word" (Mishlei 13:5). This is also what the Torah commands us: "keep far from a false matter" (Shemot 23:7). Notice that the verse did not say "guard against falsehood" but rather "keep far from a false matter", to rouse us on the great extent one must distance and flee far away from falsehood, as scripture says: "The remnant of Israel shall not do iniquity nor speak lies, and a deceitful tongue shall not be found in their mouths" (Tzefania 3:13).