Keritot, Chapter Six, Mishnah Eight
Most of this mishnah deals with the higher and lower sacrifice which is the sacrifice that is more or less expensive depending upon the wealth of the sinner. This type of sacrifice is mentioned in Leviticus 5, where there are three levels (lamb/goat, birds, grain) and Leviticus 12-14 where there are two levels (lamb/goat and birds).
1) One may bring with [money] dedicated to buy a lamb [for a hatat] a goat, or with [what was] dedicated to buy a goat [one may bring] a lamb;
a) Or with [what was] dedicated to buy a lamb or a goat [one may bring] turtle-doves or young pigeons;
b) Or with [what was] dedicated to buy turtle-doves or young pigeons [one may bring] the tenth of an ephah.
2) How so? If a man set apart [money] for a lamb or a goat [for a hatat] and he became poor, he may bring a bird-offering;
a) If he became still poorer he may bring the tenth of an ephah.
3) If a man set apart [money] for the tenth of an ephah and he became richer, he must bring a bird-offering;
If he became still richer he must bring a lamb or a goat.
4) If a man set apart a lamb or a goat and they became blemished, he may bring with their price a bird-offering;
a) But if he set apart a bird-offering and it became blemished, he may not bring with its price the tenth of an ephah, since a bird-offering cannot be redeemed.
Section one: If one sets aside money to buy a female lamb (one year old) as a hatat he can use it to buy a female goat (two years old) because both are usable as a hatat.
If he sets aside money to buy a lamb or goat and then becomes poor he can use the money to buy bird offerings, as section two explains. Similarly, if he sets aside money to buy bird offerings and then becomes even poorer, he can use the money to buy a tenth of an ephah of grain. This would only work for those who can bring such an offering, see 2:4.
Section two: This section simply explains section one.
Section three: Just as one who grows poor can use money set aside to buy an expensive offering to buy a cheaper one, so too one who becomes rich can/must use money set aside to buy a cheap offering to buy a more expensive one.
Section four: If a man set apart a lamb or goat and then became poor and the animal became blemished, he may sell the animal and use the money to buy a cheaper offering. However, if he sets aside a bird offering and it loses a limb (this counts as a blemish for birds) and he becomes poor enough to offer a grain sacrifice he cannot sell it and use its price to buy grain, because there is no way to redeem bird sacrifices. The verses about redeeming a sanctified animal (see Leviticus 27:11-13) refer only to a beast (cow, sheep or goat). Bird offerings can never be redeemed.