Is it a mitzvah or a sin?
And Moshe spoke to the heads of the tribes, to the children of Israel, saying: This is the thing that Hashem commanded. A man who makes a vow… (30:2-3)
Rashi says, quoting Bava Basra 120b, that Moshe taught the parsha about vows not only to the heads of the tribes but to all of Israel. This is derived from the words "this is the thing" which appear here and in Vayikra 17:2, in the section dealing with the prohibition to offer sacrifices outside the Temple. There it states explicitly, "Speak to Aharon and to his sons and to all of Israel."
But if so, why does the Torah mention the heads of the tribes? The Gemara derives a law from this, but what is the simple meaning? Furthermore, what is the significance of the fact that this is derived from the parsha of sacrifices outside the Temple, which seemingly has nothing to do with vows? And what is the significance of the use of the words "this is the thing" for the derivation?
The Satmar Rav explained: sometimes the evil inclination induces a person to sin by convincing him that the sin is actually a great mitzvah. A vow, for example, seems to be a mitzvah – to spur oneself higher in the service of G-d - but actually it is a sin. The same is true of sacrifices outside the Temple. Chazal highlight this similarity when they say, "One who vows is as if he built an unauthorized altar, and if he fulfills his vow it is as if he offered a sacrifice on it" (Nedarim 22a). This is why these two passages are connected by their use of the same key words, "this is the thing." The Torah is telling us: This is the thing that Hashem commanded – this and nothing else. Do not add to the Torah, placing upon yourself new prohibitions or bringing offerings in places other than the Temple.
This is why the heads of the tribes are mentioned. There are times in life when it is unclear to us what is a mitzvah and what is a sin. Then we must turn to the heads of the tribes – the leading rabbis of our time – and ask them what to do. (Toros V'uvdos Mibeis Raboseinu, p. 339)