Or HaChaim on Deuteronomy 23:24/אור החיים על דברים כ״ג:כ״ד

1 מוצא שפתיך תשמר, "You shall carry out what you have promised with your lips." The Torah suggests that you should wait before making a vow, a promise, until you are in a position to honour it forthwith. In that event it is perfectly acceptable for you to make vows. This is why the Torah writes immediately adjoining ועשית כאשר נדרת, "and you will do in accordance with what you have vowed." The famous teacher of the Mishnah Hillel conducted himself according to this principle by not designating the lamb he would offer as a passover offering as such until he had reached the Temple yard where it was to be slaughtered (compare Pessachim 86).

2Another meaning of our verse may be this: the words מוצא שפתיך תשמר mean that you should be very careful before you say anything (as it may be in the nature of a commitment). If, however, you did say something, treat it as if it were as sacred as a vow, i.e. כאשר נדרת לשם אלוקיך נדבה, "as if you had made a sacred vow to the Lord your G'd, instead of merely a voluntary gift of a secular nature." The Torah uses the expression נדבה to indicate the generous frame of mind you were in when you made this promise. We know that this is the general meaning of the word נדבה from Exodus 25,20: "which he donates in his heart," or Exodus 35,21: "which his spirit had donated."

3אשר דברת בפיך, "as you have said with your mouth." We may understand this in light of Zohar volume three page 294 commenting on Kohelet 10,20: "for a bird of the sky may carry the sound, etc." When a man merely entertains some thoughts, his lips already move without his being aware of this. This is what the Torah alludes to with the words: "which you uttered with your mouth." If the meaning were not what the Zohar suggested what point is there in the Torah adding the word: "with your mouth?" Who does not know that one speaks with one's mouth? Our sages in Rosh Hashanah 6 interpret our verse in a different manner. After all, there are 70 legitimate ways of interpreting the written Torah.