Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)By the Chofetz Chaim

This book lists the Torah mitzvos that can be observed today

Second Section – the Negative Commandments

109. It is a negative commandment “do not redeem the firstborn of a kosher animal”

As it states in Bamidbar (Numbers) 18:17, “but the firstborn of an ox… do not redeem.” During this time when the Temple is not standing, it is allowable to sell the firstborn ox, but the person who buys it must treat it in accordance with the holiness of a firstborn animal. A firstborn ox that has a blemish is allowed to be sold – no matter if it is alive or dead – as long as it is not sold in a meat market.

This applies to men and to women.