Or HaChaim on Numbers 30:3/אור החיים על במדבר ל׳:ג׳

1 איש כי ידור נדר, When a man vows a vow, etc. Why did the Torah repeat both the word נדר, vow, and the word שבעה, oath? Why could the Torah not simply have written איש כי ידור לה׳ או ישבע? Furthermore, why did the Torah phrase observance both in a positive and in a negative connotation, i.e. "he must not profane his word, as well as he must do in accordance with what came out of his mouth?" I believe we must look for the key to the way the Torah worded this legislation by remembering that there are two categories of vows and oaths. One category is related to a person's soul, i.e. he may undertake to study Torah, give a certain amount of charity, refrain from abominable practices, not to visit places predominantly visited by women, etc. The other category of vows and oaths pertains to the body and involves matters which are neither forbidden to us nor are we commanded to do them. Here too there are two divisions; one may vow to eat certain things, drink certain liquids, or live in a certain house, etc. Or, one may undertake on oath not to eat certain things, drink certain liquids, or wear certain clothes. G'd legislated here how a person should best conduct himself. The words כי ידור נדר, i.e. an ordinary vow involves an undertaking to go to a certain place or to perform a certain task. The addition of the word לה׳ means that different rules apply when the vow is not of a secular nature but relates to one's conduct vis-a-vis G'd. The addition of this word therefore means that the word נדר has not been repeated at all, but that once it applies to secular vows and once to religious vows.

2The Torah goes on או השבע שבעה, "or he swears an oath, etc." In the first instance the oath concerns something one undertakes not to do as stated here לאסור אסר על נפשו, "to deny himself something." Here too we have two categories of oaths. One may swear not to do something which is already forbidden, as Boaz did when he wanted to reinforce his resolution not to sin by swearing an oath.

3The Torah was very astute in separating the word "vow" from the word "oath" by interposing the word "to G'd," to alert us to the fact that the reason both the word נדר and the word שבעה have been repeated is to allude to vows or oaths of a religious nature. Such religious vows or oaths are not subject to the same rules which govern vows or oaths of a secular nature. The words לא יחל דברו, "he must not profane his word," refer to vows and oaths which involve abstaining from something of a purely secular nature such as Boaz' oath; the rule which applies is: "he must not break his word; however, a collegium of judges has the power to annul the vow or oath in question (compare Nedarim 14). With respect to vows involving matters of religious significance, such as declaring something sacred for use only by the Temple treasury, etc., or oaths of a religious nature, the words which are applicable are: "everything which his mouth has uttered he shall do."

4Another reason why the Torah wrote the word לשם can be understood in light of Maimonides Hilchot Nedarim chapter 11. He writes that if a 12 year old boy has made a vow or sworn an oath, we examine him to find out if he is mature enough to have understood the significance of his undertaking, i.e. "to whom he made this undertaking." If so, he is held responsible just as an adult. The exegetical tool for this is the word איש before the legislation. The legislation applies to anyone who qualifies intellectually under the heading of איש, is mentally of age and realises his obligations vis-a vis G'd.