וְאִם־לֹא֩ תַשִּׂ֨יג יָד֜וֹ לִשְׁתֵּ֣י תֹרִ֗ים אוֹ֮ לִשְׁנֵ֣י בְנֵי־יוֹנָה֒ וְהֵבִ֨יא אֶת־קָרְבָּנ֜וֹ אֲשֶׁ֣ר חָטָ֗א עֲשִׂירִ֧ת הָאֵפָ֛ה סֹ֖לֶת לְחַטָּ֑את לֹא־יָשִׂ֨ים עָלֶ֜יהָ שֶׁ֗מֶן וְלֹא־יִתֵּ֤ן עָלֶ֙יהָ֙ לְבֹנָ֔ה כִּ֥י חַטָּ֖את הִֽיא׃
Но если его средств недостаточно для двух горлиц или двух молодых голубей, то он принесет свою жертву за то, в чем он согрешил, десятую часть эфы мелкой муки для жертвы за грех; он не должен наливать на него масла и не ставить на него ладан; ибо это жертва за грех.
Rashi
כי חטאת הוא [HE SHALL PUT NO OIL UPON IT, NEITHER SHALL HE PUT ANY FRANKINCENSE THEREON:] FOR IT IS A SIN-OFFERING — a sinner’s offering, and therefore it is not proper that his offering should be embellished by oil and frankincense (Menachot 6a).
Rav Hirsch
V. 11 את קרבנו אשר חטא. Vergleichen wir Stellen wie ואת אשר חטא מן הקדש (Kap. 5, 16) חוטא נפשו (Prov 20, 2) וחוטא נפשך (Habakuk 2, 10), so erkennen wir, daß das Verbum חטא nicht nur das Vergehen, das חטא, sondern auch den Gegenstand als Objekt nach sich haben kann, gegen welchen man sich vergangen, den man durch sein Vergehen verletzt. את קרבנו אשר חטא heißt daher: den Ausdruck der Gottesnähe, die er durch sein Vergehen verscherzt hat.
Chizkuni
One tenth of an ephah. A commandment performed at its proper time is more welcome to the Lord than at a later time, i.e. the sinner having expected some substantial money coming his way and therefore preferring to wait with the sin offering until he can present an animal of greater worth. The same rule applies to the poor person who, although not even having sinned, promises his own worth as a donation to G-d. It is preferable that he donates one sela (or whatever the Kohen determines he can afford) rather than wait until he becomes rich and give 50 shekalim. (See Leviticus 27.2-8)