Nedarim 13A

דמחית בשר בכור ומחית בשר דהאיך גביה, ואמר ״זה כזה״. ותנאי היא.

that he places the meat of a firstborn animal in one place and he places another piece of meat next to it, and he said: This second piece of meat is hereby like that meat of the firstborn animal, and it is a dispute between tanna’im about whether he is referring to the original forbidden status of the firstborn animal or its current permitted status?

לא, דכולי עלמא לפני זריקת דמים, ומאי טעמא דמאן דשרי ⁠— אמר קרא: ״כי ידר״, עד שידור בדבר הנדור, לאפוקי בכור, דדבר האסור הוא.

The Talmud responds: No, everyone agrees that he is referring to the status of the animal before the sprinkling of the blood, and what is the reason of the one who renders it permitted? The verse states: “When a man takes a vow” (Numbers 30:3), which indicates that he has not done so until he takes a vow with an item forbidden by means of a vow, i.e., by extending the status of an item that itself was forbidden by a means of a vow. This comes to exclude a firstborn, which is an item that is forbidden by the Torah rather than by means of a vow; a firstborn animal is sacred simply because it is born first.

ומאן דאסר, אמר קרא: ״לה׳״, לרבות דבר האסור.

And the one who renders it forbidden holds that it is because the verse states: “To YHWH” (Numbers 30:3), which comes to include one who takes a vow by associating an object with an item that is forbidden by the Torah.

ומאן דשרי, ״לה׳״ מאי עביד ליה? מיבעי ליה למתפיס בחטאת ואשם.

The Talmud asks: And the one who renders it permitted based on the principle that one cannot take a vow by associating the item with an item that is forbidden by the Torah, what does he do with the expression “to YHWH”? The Talmud answers: He requires this expression to include the case of one who associates the object of his vow with a sin-offering and a guilt-offering. Although these offerings are obligatory rather than voluntary, and in that regard they are dissimilar to items forbidden by means of a vow, one can render another item forbidden by associating it with these offerings.