Based on a Naaleh.com class on Chassidut
In Parshat Matot, Moshe speaks to the Roshei Matot-the tribal leaders, regarding the positive and negative commandments related to oaths and vows. Why did Moshe speak to the Roshei Matot first and not directly to the Jewish people? Rashi explains that if a person regrets the oath that he uttered in a fit of emotion, he can go to a great Torah scholar, such as one of the tribal leaders, and have the vow rescinded. Therefore, Moshe spoke to the Roshei
The Avnei Nezer questions Rashi’s explanation. If any Jew can make a vow, why does the Torah specifically single out the tribal leaders? He answers that it seems as if taking a vow could be a violation of “Lo Tosef”-the prohibition not to add any new mitzvot to the Torah. How can the Torah give a license to make a vow, thus creating additional obligations? The Avnei Nezer answers that the mitzvot of the Torah are G-d given, and are therefore eternal. In
contrast, oaths are initiated by man, and can be rescinded by the Sages of Israel. Therefore, because the concept of hatarat nedarim authorizes vows, Moshe addressed the tribal leaders, who can nullify oaths, first.
The Shem MiShmuel asks, how can Man have the power to make vows and create personal prohibitions? He explains that a Jew is a holy vessel endowed with angelic sanctity. Speech comes from within a person, taking on the kedusha of the human soul. Because speech is holy, vows and promises are endowed with special sanctity and must be fulfilled. This is the secret of the power of a neder.
What if our speech has been defiled, however? How can we say that our vows have sanctity, if we have desecrated our speech? Even if an individual has sinned in the area of speech, our Torah sages and righteous people share their great holiness with every Jew, enabling the individual to tap into their holiness and imbue his words with spiritual power. Therefore, Moshe spoke first to the Roshei Matot-the righteous leaders of Israel, signifying that their sanctity in tandem with the eternal kedusha of Knesset Yisrael will lend holiness to all vows and oaths.