Hashem Doesn’t Change His Words

אִ יׁש ּכִ י יִּדֹר נֶדֶ ר לַה' אֹו הִ ּׁשָ בַע ׁשְ בֻעָה לֶאְ סֹר אִ ּסָר וגו' לֹא יַחֵל ּדְ בָרֹו ּכְ כָל הַּיֹצֵא מִ ּפִ יו יַעֲׂשֶ ה (ל, ג)

When a man promises a promise to Hashem… he shall not violate his word; according to whatever came out of his mouth, he shall do. (30:3)

Hashem Doesn’t Change His Words:

The Abir Yaakov of Sadiger zy”a explains the words “ish ki yador neder l’Hashem” to mean that a person should make his body into a “mador” (dwelling place) for Hashem. One can do this by “not violating his words”, i.e., being careful with every word that comes out of his mouth. If one does this “whatever comes of his mouth shall be done”, meaning that whatever he asks Hashem for will be granted to him.