Side-ring EXTRA · Means-based substitution
The Torah’s own “which his means allow” (אשר תשיג ידו) parallel to other graded offerings; classical treatment in Toras Kohanim / Sifra on graded offerings.
English: “Which his means allow” — according to his capacity he brings: the wealthy with livestock, the poor with birds.
Graded offering
The Torah scales the purification offering to means so that poverty never blocks purification, while the asham and oil rite remain the same for rich and poor.