The Talmud explains: The Mishnah states two different principles with regard to one who vows to provide the offering of a metzora on behalf of another. One principle applies to a destitute metzora for whom another destitute person takes a vow to provide his offering. In that case, he provides the offering of the destitute. And the one other principle, i.e., the statement that the halakha is different in the case of offerings, serves to exclude the case of a wealthy metzora for whom a destitute person takes a vow to provide his offering. In that situation, although the one who took the vow is destitute, he must provide the offering of a wealthy person.
The Talmud explains why this last ruling is necessary. It might enter your mind to say: Since the destitute were included in the verse: “And his means do not suffice,” with regard to the leniency which allows them to bring the offering of the destitute when taking vows to provide for destitute lepers, they should likewise be included in this leniency when providing for all lepers, even wealthy ones. Consequently, the tanna teaches us that this leniency does not apply when the lepers are wealthy.
לפי שמצינו בערכין, עני שהעריך את העשיר נותן ערך עני, יכול אף זה כן, תלמוד לומר ״אם דל הוא״.
Similarly, it is taught in a baraita: Since we find with regard to valuations that a destitute person who valuated a wealthy person gives the valuation in accordance with the means of a destitute person, one might have thought that in this case, where one takes a vow to provide the offering of a wealthy metzora, the halakha is also so. Consequently, the verse states: “And if he is poor,” from which it is derived that the offering of the destitute metzora is brought only when the metzora himself is destitute.