וכל פטר חמור תפדה בשה
שמות יג יג
What
To redeem every first born male donkey with a lamb
How
The Yisrael takes a lamb and gives to the Kohen in redemption of the donkey. The donkey belongs to Hashem as explained in Mitzva 18, Hashem gives it to the Kohen. Therefore the Yisrael must redeem it from the Kohen. If he doesn't have a lamb, he may redeem it with the value of a lamb. The lamb is not Kadosh in the hands of the Kohen, nor the donkey in the hands of the Yisrael.
Type
Positive
When
Anytime
On condition
That one has a first born.
Who
Males and females. Yisraelim but not Kohanim, Leviim .
Additional details
In Masechta Bechoros
Why
So that we always remember the miracle Hashem did for us in Egypt, that he killed the first born of the Egyptians who are compared to donkeys.