14-5771 Mitzvah N-109
Negative Mitzvah 109 – This is a negative commandment: do not redeem the firstling of a pure (kosher) animal.
Hafetz Hayim – As Scripture says: “but the firstling of an ox… you shall not redeem” (Numb. 18:17). At the time that the Sanctuary is not in existence, it is permissible to sell it; but the buyer has to treat it in accordance with the holiness of a firstborn animal. As for a firstling with a disfiguring defect, it is permissible to sell it, whether it be alive or ritually slaughtered, provided it is not sold at the meat-market. This is in effect, everywhere and in every time for both men and women.
This is also not a law for vegetarians or vegans.
The law of the Torah is that the firstborn of kosher animals is to be sacrificed on the alter. Parts of the animal are then eaten by the priests who are officiating. The rest is burned on the alter. Non-kosher animals were not permitted to be sacrificed and they had to be redeemed so that they could be used for other purposes. The owner would pay a priest a set price and the animal would lose its sanctity as a first born. Kosher animals had to be sacrificed and could not be redeemed. They belonged to the priests. (The firstborn son is also holy and must be redeemed; this is the origin of the Pidyon HaBen ceremony). If the firstborn animal had a defect that would not permit it to be sacrificed, then that animal was to be redeemed and the money paid to a priest. We should see this as a livestock “tax” that was paid to the priests from the firstborn of cattle (oxen, sheep and goats).
That was the law if the Temple was still in existence. Since the Temple was destroyed, the animals can no longer be sacrificed and therefore we need to handle them differently. The rule here is that we are permitted to sell a firstborn animal, but a Jew who buys it should not put it to work like any other beast of burden. It is a holy animal and should not be plowing or threshing. If the animal has a defect it can be sold and used as normal. One can use it for personal consumption but should not sell it for its meat to a butcher. This is how it is handled differently from other animals.
I must admit that this is not a mitzvah that is of use to anyone other than those engaged in animal husbandry. This would be a special area of Jewish Law and I am not an authority in this area. It is interesting how these firstborn animals were handled differently in ancient times and how we should handle them differently today. I don’t see any reason to go into this in more detail than we have.