EXOD185 Redeem the firstborn of a donkey. If the firstborn of a donkey is a male, it cannot be used for any purpose until it is redeemed. The redemption is onto a lamb, and if the owner has no lamb he uses money instead--equivalent to the value of a lamb. So that Jews will never forget the great miracle that Hashem performed for them just prior to the Exodus from Egypt when he killed Egypt’s firstborn males. The men of Egypt are likened to donkeys.