Why does the Torah here write "Al Chataso asher Chata" whereas by Ashirus (in Pasuk 6) and by Dalus (in Pasuk 10) it writes "me'Chataso"?
Rashi: To teach that if the sinner became poor after setting aside money to purchase a lamb or a goat, he may use some of the money to purchase two birds; 1 and similarly, if after setting aside money to purchase two birds, he becomes poorer still, he may even use some of the money to purchase flour for a Minchah ("me'Chataso"). Whereas in the event that he designated money to purchase a Minchah, and he became rich, he is obligated to add money ("al Chataso" and to purchase accordingly.
What is "me'Achas me'Eileh coming to teach us?
Rashi: It teaches us that the Korban depends on which Korban he can afford - irrespective of whether he transgressed one of severe, light or very light 1 sins listed here.
Rashbam: It means that the Korban comes to atone for one of the three sins mentioned earlier - "v'Sham'ah Kol Alah", 'Shevu'as Bituy' and 'Tum'as Mikdash ve'Kodashav'.
Mizrachi: Tum'as Mikdash v'Kadashav is servere, because it has Kares (for Mezid). Shevu'as ha'Edus is light - it has no Kares, but the Korban is even for Meizid. Shevu'as Bituy is very light.
What is the Torah coming to teach us when it writes "v'Haysah la'Kohen ka'Minchah"?
Rashi #1: To teach us that the leftovers of the Minchas Chotei are eaten by the Kohanim. 1
Rashi #2 (citing Menachos, 76b): To teach us that if the sinner is a Kohen, his Minchah is completely burned on the Mizbe'ach, and that none of it is eaten. 2
Rashi writes that one might have thought that for light sins he brings flour and for severe sins an animal. It says "if he cannot afford"!
Bartenura: One might have thought that for light sins he always brings flour, and for severe sins, if he can afford an animal
Rashi writes that the change from "me'Chataso" to "Al Chataso" that if one set aside money for his Korban, and his wealth changed, he may use the money towards the new Korban that is now proper for him. How can Menachos 101b expound from this that the Minchah may be redeemed even when it is Tahor? It is needed for like Rashi said!
Moshav Zekenim (4:35): I learn from me'Chataso written about an Ashir and Dal that one may switch from an animal to a bird, and the same applies between a bird and a Minchah, or vice-versa. Al Chataso teaches that the Minchah may be redeemed even when it is Tahor. 1
Moshav Zekenim: Why do we need me'Chataso written about an Ashir? If one may switch from a bird to a Minchah, even though it is not alive, all the more so one may switch from a bird to an animal! There is a reason why we cannot learn one from another; the Gemara did not bother to teach why.