Why is the Torah lenient on the sinner here, allowing him to bring a smaller Korban if he cannot afford a bigger one?
Ramban: In the two cases of Shevu'ah, it is because they are not subject to Kares; Whereas in that of Tum'as Mikdash ve'Kodashav, it is due to the fact that he erred with regard to a Mitzvah (either whilst eating Kodshim or whilst going to the Beis-ha'Mikdash to bow down to Hashem or to bring a Korban). 1
Da'as Zekenim, Hadar Zekenim (4:3), Moshav Zekenim: He did not profit from his Aveirah, e.g. entering the Mikdash b'Tum'ah or a false Shevu'as Bituy 2
Moshav Zekenim: This is difficult, for if one had a baby to circumcise after Shabbos, and circumcised him on Shabbos, all agree that he is liable (Chatas Behemah, even if he is poor), since he had no Mitzvah to do [that overrides Shabbos - Shabbos 137a]!
Hadar Zekenim, Moshav Zekenim; One who has two liabilities - he took from Hekdesh and benefited, or swore falsely and benefits (keeps the money), he brings an Asham for at least two Sela'im. The same price applies to Asham Taluy. Since one is lenient about Safek sin, the Torah is stringent and obligates him an expensive Korban. (One who ate Kodshim b'Tum'ah, he benefited. Why does he bring Oleh v'Yored?- PF)
Why does the Torah require him to bring both Olas ha'Of and Chatas ha'Of?
Moshav Zekenim citing the Rosh, Ramban (citing Ibn Ezra): They are in place of Chatas Behemah, in which part goes [on the fire on the Mizbe'ach] to Hashem, and part goes to Kohanim. Nothing from Chatas ha'Of goes to Hashem, and Kohanim do not get anything from Olas ha'Of,