Musar על במדבר 30:3

לא יחל דברו, ככל היוצא מפיו יעשה . The last four words in this verse appear superfluous, since if we are prohibited from breaking a vow, we must obviously do according to what we have vowed. The moral lesson the Torah wishes to convey is therefore not connected to the laws of vows and oaths, rather the Torah emphasizes that we must keep our word even if what we have said was not said in the form of a promise or vow. Moses told the tribes of Gad and Reuben: "Carry out what you have said" (32,24). Rashi comments that Moses referred to their undertaking to be the spearhead of the Jewish army in their conquest and division of the Land of Canaan west of the Jordan river (32,18). The latter part was something that Moses had not demanded of them. Another statement of intent had been volunteered by those tribes which had not been part of the condition Moses imposed upon them in order to retain their possessions east of the Jordan.

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.

For the heart of his fellow relies upon him and trusts in him, inasmuch as the gift in question is so small. And if it is a poor man to whom he made the promise, even though the gift mentioned was a large one (for which reason the poor man should have doubted the promise) and he goes back on his word, then his evil is very great, for he has made a vow and transgressed the commandment, "He shall not break his word" (Num. 30:3). And thus it is with one who boasts before many that he will give a gift to a certain man, this is very much like a promise, for he boasts of his generosity, and therefore it is not right that he should go back on his word after he has honored and praised himself in this matter.