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By the mitzvah of keeping your promise it says Moshe told the heads of each Shevet about this mitzvah. Why does the torah write this by this mitzvah? Bais Din has the obligation to enforce every mitzvah. The Netziv says that we usually make a promise under some sort of duress in the hopes that our promise will help us achieve our goal whether it is a promise to Hashem to save us or help us in some way or to another person to gain something in return from that person.

Unfortunately, he says, keeping promises is often not easy for us. Typically when the duress has passed and it is time for us to fulfill our end our enthusiasm dramatically wanes and we come up with all sorts of rationalizations why we need not keep our exactly as we promised it or even not at all. This is why Moshe told the leaders of the people that it is up to them to make sure that people keep their promise.

Using the same explanation Rav Zvi Hersh Voidaslav father of Rav Simcha Bunim MiPeshischa explains why the Torah says the seemingly superfluous words "K'Chol Hayotzeh MiPiv Yaaseh"; its deeper meaning is that you should keep your promise with exactly the same enthusiasm that you made it.
http://www.thejewisheye.com/rev_pmatos.html