Positive commandment · Mitzvat Aseh #72 of 613
Sacrifice that varies up and down
להקריב קרבן עולה ויורד
The verse · Vayikra 5:7
וְאִם־לֹא תַגִּיעַ יָדוֹ דֵּי שֶׂה וְהֵבִיא אֶת־אֲשָׁמוֹ אֲשֶׁר חָטָא שְׁתֵּי תֹרִים אוֹ־שְׁנֵי בְנֵי־יוֹנָה לַה׳ אֶחָד לְחַטָּאת וְאֶחָד לְעֹלָה
“And if he be not able to bring a lamb, then he shall bring for his trespass, which he has committed, two turtledoves, or two young pigeons, to the Lord; one for a sin offering, and the other for a burnt offering.”
The Rambam’s words
To offer "the adjustable guilt offering" [as atonement for the violation of certain transgressions], as [Leviticus 5:1,11] states: "If his means are not sufficient.... If his means do not suffice...."
להקריב קרבן עולה ויורד, שנאמר: "ואם לא תגיע ידו" (ויקרא ה,ז), "ואם לא תשיג ידו" (ויקרא ה,יא).
Why · Sefer HaChinuch #123
Sacrifice that varies up and down
“From the roots of the commandment, we have already said (Sefer HaChinukh 95) that the matter of the sacrifice is to remind — and to have the thinker place in his heart — by force of action that he made his deeds bad; and that he request forgiveness about the past and be careful about the future.”
Who & when
“And [it] is practiced at the time of the Temple by males and females; except for the sacrifice of the oaths of testimony, which is not practiced by females — since they are not in the category of testimony, as we have written above (Sefer HaChinukh 122).”
Where in the Mishneh Torah
Hilchot Offerings for Unintentional Transgressions הלכות שגגות