If only a donkey's firstborn is redeemed, what does the Torah mean when it says in (Numbers 18:15), "but redeem shall you redeem the first-born of the unclean beast"? The Mekhilta already established that this cannot refer to redeeming other unclean animals' firstborns — that obligation belongs exclusively to the donkey. So what does the verse teach?
The answer involves a creative legal application. When a verse cannot apply to its most obvious context, the rabbis redirect its teaching elsewhere. Since it cannot refer to firstborn redemption, the verse must be teaching something else entirely: that a person may dedicate an unclean animal to the Temple treasury for maintenance purposes, and then redeem it with money.
This is called "bedek habayit" — the department responsible for Temple upkeep. A person could vow to give the value of their donkey, camel, or horse to the Temple. The animal itself would then be redeemed — purchased back with coins — and the money would go toward maintaining the sacred building.
The Mekhilta's reasoning here demonstrates a fundamental rabbinic principle: no verse in the Torah is superfluous. If a passage cannot mean what it appears to mean on the surface, it must be teaching a different law entirely. Every word carries legal weight, and the rabbis refused to let a single phrase go to waste.