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
First Section – the Positive Commandments
55. It is a positive commandment to redeem a firstborn male donkey for a lamb
As it states in Shemos (Exodus) 34:20, "A firstborn of a donkey redeem with a lamb." The lamb should be given to the Cohain as it states in Bamidbar (Numbers) 18:15, "All firstborns... are yours." A Cohain and Levi are not required to do this. If a person has no lamb to redeem it on, he should redeem it for money and give the money to the Cohain. If the firstborn donkey is worth 10 selaim, he can redeem it with one dinar, but a Cohain and Levi are not required to do this either.
This applies in all places and at all times for men and for women.