“Redeem the firstborn of a donkey with a lamb”
You must redeem the firstborn of a donkey with a lamb; but if you do not redeem it, you are to break its neck. You must redeem all the firstborn of your sons. (Exodus 34:20 BSB)
What it sounds like today
A bizarre rule about breaking a donkey's neck or swapping a lamb.
The cultural key
The strange-sounding law follows from the Passover. Because God spared Israel's firstborn when He struck down Egypt's, every firstborn male, of people and of livestock, now belonged to Him (13:2). A firstborn clean animal (a lamb or calf) could simply be offered in sacrifice. But a donkey was ritually unclean and could not be sacrificed, and yet it still 'belonged' to God, so it had to be redeemed, bought back for ordinary use by substituting a lamb in its place; if the owner refused to redeem it, he forfeited it entirely (its neck was broken), gaining nothing. Crucially, the same principle applied to firstborn sons: they too belonged to God, but they were never sacrificed, they were redeemed with a payment (Numbers 18:15-16), which sharply set Israel apart from neighbors who practiced child sacrifice. The whole system teaches substitution, a life bought back by another. A modern reader sees an odd animal rule; the first hearers heard the logic of Passover, redemption by a substitute, extended over all of life.
Recovered meaning
Because the Passover spared Israel's firstborn, every firstborn belonged to God; an unclean donkey (unfit for sacrifice) had to be bought back with a lamb, and firstborn sons were redeemed with a payment, never sacrificed. The law drills in redemption by substitution.