A firstborn becomes sacred when born to an animal belonging to a Jew. Even jointly owned animals produce a sacred firstborn, as long as all the owners are Jewish. If the animal belongs to a non-Jew, or if a non-Jew holds any share in it, the firstborn has no sanctity.
With the Temple destroyed, the proper practice is to sell a share in animals that have not yet given birth to a non-Jew, so their firstborns will not become sacred. The mitzva is to bring the firstborn as an offering — impossible without the Temple — yet firstborns may not be slaughtered or put to work. They must be cared for until a blemish appears, which for most animals never happens before death. Kohanim would face years of unrecompensed care, a burden that could lead some to inflict blemishes deliberately or to use the animals despite their sanctity. The Rishonim therefore instructed that a non-Jew be made a partner before birth, so the firstborns will not become sacred. Though this sale circumvents the mitzva, doing so is preferable to entering a situation likely to lead to transgression (Tosafot, Bekhorot 3b; SA YD 320:6).
For the partnership to be effective, the sold limb must be one whose removal would be significant — for example, the esophagus or the ears. Selling such a limb of every animal that has not yet given birth ensures no firstborn of those animals will be sacred (SA YD 320:4).
The sale must be executed through a mode of acquisition effective between a Jew and a non-Jew under Torah law. Since the Rishonim disagree on which form applies, the sale must satisfy both views: payment of money, per Rashi, and meshikha (pulling into one’s domain), per Rabbenu Tam. The non-Jew pays one peruta for his share; even though the limbs are worth more, the Jew’s intent to sell them cheaply in order to avoid the firstborn’s sanctity makes the sale valid. The non-Jew must also perform meshikha by drawing the animals into his domain; if he owns no land there, the Jew rents him the place where the animals stand, and in this way the non-Jew acquires his share (SA YD 320:6).
The poskim disagree about whether the sale can be executed through the legally recognized form of acquisition under civil law. Most hold that since “the law of the kingdom is law,” a legally valid acquisition makes the non-Jew a genuine partner and the firstborns non-sacred (Knesset Yeḥezkel §14; Ḥatam Sofer, YD §314). Some hold that only a Torah-valid acquisition suffices (Divrei Ḥayim 2:117). Ideally, one satisfies both views by also executing the sale through a Torah-valid mode.
