TT #48 · Parshas Tazria · Aliyah 1 · Passuk 7
English: He shall offer it before GOD and make expiation on her behalf; she shall then be pure from her flow of blood. Such are the rituals concerning a woman who bears a child, male or female.
Chabad.org — Chumash, English translation + Rashi for Vayikra 12:7
Torah Temimah cites Sifra (Toras Kohanim) for identifying which of the two offerings remains indispensable when the pair is incomplete.
Sifra — Vayikra 12, Midrash Sefer
English: The text has established that one of the two offerings is indispensable, but it has not yet identified which one. The words “and he shall make expiation” show that the sin offering is indispensable, as expiation is associated throughout the Torah with the sin offering.
Textual limitation
The wording “and he shall make expiation” identifies the sin offering as the indispensable offering.