When a sin offering (חטאת -chatat) can be voluntary TB Nazir 25

Normally nobody thinks of a sin offering (חטאת -chatat) as something voluntary. A person may only bring a sin offering after committing a sin. “A sin offering is an offering to atone for and purge a sin. It is an expression of sorrow for the error and desire to be reconciled with God. The term for this type of offering is chatat, from the word chayt, meaning ‘missing the mark.’ A chatat could only be offered for unintentional sins committed through carelessness, not for intentional, malicious sins. The size of the offering varied according to the nature of the sin and the financial means of the sinner. Some chatatot are individual and some are communal. Communal offerings represent the interdependence of the community, and the fact that we are all responsible for each other’s sins. A few special chatatot could not be eaten, but for the most part, for the average person’s personal sin, the chatat was eaten by the kohanim.”  (https://www.jewishvirtuallibrary.org/sacrifices-and-offerings-karbanot)

To appreciate the Gemara’s question on today’s daf TB Nazir 25, we have to know that once an animal has been designated as a chatat and is no longer required (for example, the person who needs to bring the chatat has died before offering it), the animals is brought to the Temple and is starved to death because is no longer fit to be offered up on the altar. It can’t be substituted for any other type of sacrifice or for anybody else’s chatat. If you remember when we studied massekhet Shekalim, there were 13 pushkes (boxes to collect tzedakkah) in the Temple. Seven of them were for the purpose of buying whole burnt offerings (olot- עולות) to offer up on the altar when there were no other sacrifices to be offered up. How can money for a nazir’s chatat be donated to purchase and olah?

“§ It was stated above, with regard to one who separated money for his naziriteship and then died, that if he had unallocated funds, they are all allocated for communal gift offerings. The Gemara asks: But the money for the sin-offering is mingled with these unallocated funds. How can it all be used for gift burnt-offerings?[1]...Reish Lakish said: The verse states: “Whether for any of their vows or for any of their gift offerings, which are brought to the Lord as a burnt-offering” (Leviticus 22:18). In this verse, the Torah states that the surplus of a vow (motar neder-מוֹתַר נֶדֶר) shall be for a gift offering. Here too, if the vow of naziriteship cannot be fulfilled, due to the death of the one who took the vow, the money is for a gift offering.” (Sefaria.org translation) An example of a surplus of a vow is when there is deflation. If a person designates $100 which is the normal cost of such an animal to purchase voluntary sacrifice, but when he actually goes to buy the animal it only costs $80. The extra $20 is considered the surplus of a vow (motar neder-מוֹתַר נֶדֶר) and is donated for the purpose of buying olot.

Tosefot ד"ה רֵישׁ לָקִישׁ אָמַר: "לְכׇל נִדְרֵיהֶם וּלְכׇל נִדְבוֹתָם", הַתּוֹרָה אָמְרָה: מוֹתַר נֶדֶר יְהֵא לִנְדָבָה. Explains how Reish Lakish arrives at the conclusion that the nazir’s chatat is unique. No one is obligated to become a nazir. The person voluntarily takes an oath to be a nazir. Because of this voluntary oath all of the nazir’s sacrifices are considered to be voluntary, even the chatat! Consequently, that unallocated money for the nazir’s sacrifices can all be donated for the purchase of olot.

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[1] For sake of completeness Rabbi Yoḥanan provides a different solution to this question. He says that this is just a law that is traced back to Moses from upon Mount Sinai.