The How of a Vow
"You shall keep and perform what has gone forth from your mouth, according to what you have (neder) vowed to Hashem, your G-d, the (nedava) freewill offering which you promised with your mouth." (Devarim 23:24)
One can make a vow to offer an animal as a sacrifice in one of two ways. He can make a neder, which means that he obligates himself to bring an animal without designating a particular one. Or he can make a nedava, which means that he obligates himself to offer a particular animal. The crucial difference between the two is that in the first case should he set aside, after making his vow, an animal for fulfilling his promise, and that animal dies or gets lost, he has an obligation to offer another in its place. In the second case his obligation is limited to the particular animal which he promised, and if it is no longer available he bears no responsibility to replace it.
This distinction between a neder and a nedava is what causes the gemara at the very outset of the Order of Kodashim dealing with sacrifices to analyze the above-mentioned passage which speaks in the same breath of these two different kinds of vows in regard to the same animal. This passage, we are told, deals with a sacrifice which has been slaughtered while having in mind that it is being done for a different sort of sacrifice than the one on which the vow was made. Our opening mishna teaches us that in such a case the sacrifice is considered a valid sacrifice and that when its blood is subsequently applied to the altar it must be done with having in mind the type of sacrifice which the vower designated. The person who made the neder, however, is not considered as having fulfilled his obligation and must offer another animal.
We now understand this passage as teaching us that if the slaughtering was done with the proper sacrifice in mind it is considered as the fulfillment of the neder; otherwise we relate to it as a new freewill nedava offered by the original vower, which although it is a kosher sacrifice whose blood must be applied with the proper thought, does not constitute fulfillment of the neder. It also follows that if the original vow was a nedava the slaughtering of it with having another type of sacrifice in mind will not obligate the vower to replace it, because this is no worse than the animal disappearing which does not obligate him to replace it.
Omission vs. Commission
If someone unintentionally violated the Shabbat he is given an opportunity to atone for his sin by offering a chatat sin offering on the altar in the Beit Hamikdash. But what if he failed to recite the Shema or put on tefillin is there an opportunity for him to make amends by offering a sacrifice?
The answer can be found in the phrase used by the Torah in regard to the effect of the voluntary olah sacrifice. "G-d will be pleased with it," says the Torah (Vayikra 1:4), "to atone for him." Rashi, both in his commentary on Chumash and here in our gemara, cites the conclusion of our Sages that the need to please G-d referred to here is not in regard to the transgression of any prohibition, for all such transgressions are either atoned for by capital or corporal punishment, or by a sin offering. All that is left then is a sin of omission and a transgression which is not punished by lashes (because there the Torah assigned a method of undoing the damage).
Although the olah thus appears to be an atonement sacrifice for such sins, it differs from the chatat both in obligation and in nature. One who unintentionally committed a sin for which intentional violation is punished by karet (extirpation) is obligated to offer a chatat to achieve atonement. The olah, however, is a voluntary sacrifice brought by one who feels the need to please G-d after failing to do His bidding.
The other difference is that the chatat is basically an atonement which is designed to achieve forgiveness (Vayikra 4:31) while the olah is considered a gift to G-d after the sinner has repented his sin. This distinction made by the Sage Rava (Zevachim 7b) receives support from what Rabbi Shimon ruled in the case of someone who is obligated to bring both an olah and a chatat sacrifice. First comes the chatat and then the olah. This is similar to one who has offended an earthly king and wishes to make amends by offering a gift. He first enlists the services of a pleader to intercede in his behalf. Only after the pleader has done his work can the contrite offender enter with his gift. Only after the chatat has achieved forgiveness can the olah gift be offered.