Leviticus 5:1 "And if a person sins... he shall bring his guilt offering... but if he cannot afford..."

A person who is impure (see Positive Mitzvah 74) is forbidden to enter the Beit HaMikdash or eat from sacrifices. Breaking an oath or taking a false oath are also forbidden. If a person commits any of these sins, he is commanded to bring a guilt offering.

The Torah considers his financial status. A person who cannot afford an expensive offering is allowed to bring one which costs less.