Arachin, Chapter 4, Mishnah 3. This chapter is built around the principle of hasagat yad, that the Torah measures certain obligations against what a person can actually afford. Our mishnah completes a comparison that the previous mishnah began: when exactly is a person's financial state measured, and does the same rule apply across the board?
At the close of the previous mishnah we learned the law for one who took an erech vow. Such a person receives the reduced assessment of a poor man only if he was poor at two separate moments: when he made the vow, and again when he actually paid it. If he was wealthy at either point, the discount is withheld from him.
Our mishnah now tells us that this two-part measurement is unique to arachin. In its words: "Aval bekorbanot eino ken," but with korbanot it is not so.
Two Moments Versus One
With arachin, the discount hinges on his means at both ends of the process, the moment the obligation is created and the moment the payment is made. Only if he stands poor at both points does he pay the reduced amount.
Korbanot, by contrast, are gauged a single time: at the moment the liability comes into existence. Whatever his means were then determines the offering. Should he be a pauper at that point, he brings what the Torah assigns to a pauper, and money that reaches his hands later does not raise the level of what he owes.
The Metzora Who Became Rich
The mishnah's example is a metzora whose means were meager on the day his purification offerings became due, and who then came into money before he brought them. "Afilu aviv met vehiniach lo ribo": his father passes away, bequeathing him a legacy of ten thousand zuz. Or "sefinato bayam uva'u beribo'ot": a ship of his that had been sailing docks with cargo worth tens of thousands.
The verdict: "Ein lahekdesh bahen kelum," hekdesh holds no portion at all in that windfall. He discharges his duty with the offerings the Torah assigns to one of limited means, and the offerings of the wealthy are not demanded of him, since poverty was his condition when the liability attached.