Our Gemara on Amud Beis discusses the famous lomdishe concept of Bereira. Bereira is the idea of retroactive designation. For example, say you have a barrel of untithed wine, and you designate the last ounces to be Terumah, without actually separating them. The wine is now a combined mix of sacred Terumah and Chulin, but if one allows for Bereira, then the initial act of declaring the tithe falls on whatever ounces of wine are left at the end of the barrel. 

Depending on various parameters, Bereira may be valid or not, and the Halacha is that it is generally valid in regard to rabbinic prohibitions and procedures but not Biblical. Also, the Gemara (Beitzah 37b) argues that even according to the opinion that allows for Bereira on a Biblical procedure or prohibition, it cannot work in regard to an animal owned in partnership because every limb of the animal derives life and sustenance from every other limb, and so even retroactively there is no divisible designation. For example, if two partners set a different Eiruv techum boundary, their utensils and objects follow their own personal Eiruv techum boundary. Therefore, if a barrel of wine and an animal are both owned in partnership, the legal magic of Bereira could allow each partner when using the wine to have it follow his personal Eiruv Techum boundary, as if when they originally entered the partnership each one agreed that they would have ownership enacted retroactively on the portion of wine in the barrel that they are using. However, even if we would allow Bereira by objects, it will not work by an animal because it cannot be divided up, as each limb derives life sustenance from the other.

Sefer Daf al Daf quotes the Yalkut Ani (Korach) who uses these legal distinctions of Bereira to add depth to an angry prayer given by Moshe during the Korach rebellion (Bamidbar 16:15):

“Hashem, do not turn toward or accept their mincha offering.” 

It is a matter of debate amongst the commentaries (see Rashi and Ramban) as to whose offerings Moshe was referring to, whether it was Dasan and Aviram or Korach. In addition, the word mincha literally means meal offering but can also be used for other sacrifices. So, what sacrifice is Moshe referring to? From the context of the story, it would seem that he is referring to the incense that will be offered by the 250 men of the Korach contingent, and that by God not bringing fire onto their incense, it is an indication of Moshe and Aharon’s validity. Yet, this is problematic because the word used is Mincha, not ketores. The Midrash (see Rashi) holds that Moshe was insistent that they should have no share in the communal sacrifices.

According to this latter peshat, Yalkut Ani raises a question. The communal sacrifices are brought from communal funds, and so intermingled in the sacrifices were portions owned by this contingent. If a portion of the sacrifice is invalidated, then the entire sacrifice is invalidated. So how could Moshe make this request of Hashem? Yalkut Ani answers that since Bereira operates on a retroactive designation, even according to the opinions that Bereira is not valid, that is only for mortals. Since God knows the future, he knows which item will ultimately be designated, so Moshe's request was valid and Bereira could work by the sacrifices and filter out the portion owned by this sinful contingent. Even so, in regard to an animal offering, Godly Bereira is not valid because even with foresight of a retroactive designation, all parts of the animal are symbiotically joined, and are not separated. This now explains why Moshe used the word Mincha. He meant the word literally as a Meal-Offering. The discrete grain particles in a Meal Offering can be retroactively divided, unlike the body parts of the animal. Therefore Moshe was only referring to the public Meal Offerings of the Korach Contingent.

Zooming out, we should ask why would Moshe ask Hashem not to accept their offerings? Moshe who compassionately prays to Hashem to forgive the Jewish people all of the sudden prays for the opposite? Also, this hardly seems respectful to interfere with God’s relationship with another of His children. God decides whose prayers to accept, (see Shemos 32:19: “ I will be gracious to whom I choose to be gracious, and have mercy upon whom I choose to have mercy.”) Compare this to Moshe’s response in a different situation found in Bamidbar (11:29), “You are jealous on my Behalf, Yehoshua? If only all of the Nation of Hashem become prophets as I.” So long as we follow the pashut peshat, that it was the offering of the ketores at the showdown, then Moshe’s position is understandable, as there was the immediate practical need of God proving Moshe and Aharon’s legitimacy. However, if we follow the Midrashic peshat, that Moshe was referring to the public offering and the Mincha, isn’t this spiteful and disrespectful? 

Malbim (Numbers 16:21) tells us that the rebellion of Korach was different than other rebellions in that it was beyond the insubordination and disobedience, as Korach challenged the validity of Moshe’s revelation at Mount Sinai. In such a case, Moshe’s supplications that the rebellious crew be denied access to any mercy is understandable because he is defending God’s honor.