He shall not place oil on it
for it is a sin-offering
The Talmud (Shekalim 6:6) states that
the remainder of monies earmarked for sin-offerings or for guilt-offerings shall be used to purchase burnt-offerings.
The reason for this is that a sacrifice is not a bribe meant to placate Hashem to forgive ones sins. One who thinks that
way angers Hashem, Whose response is "Why do you trample My courtyards?" Hashem has no need for sacrifices, and His conduct is diametrically opposite to that of mortal man. The latter is likely to forgive one who sins against him if he presents him with a beautiful gift adorned with many adornments. Hashem, in contrast, refuses to accept an offering adorned with oil and frankincense from the sinner. Thus the Torah states: for it is a sin-offering, which
Rashi explains: and it is proper that it should not be adorned.
The reason for this is that the intention of
one who brings a sacrifice is to draw closer to Hashem, to repent of his sins and to become a different person with the proper character traits and complete faith. He must realize that his entire being belongs to Hashem, Who has given him everything, and as such it is not his "gift" to Hashem which brings about his atonement.
This is the principle of the burnt-offering.
It is improper for a sin-offering to be adorned; on the contrary, Hashem should not accept his offering at all. Only because Hashem accepts the repentance of the wicked does He accept this sacrifice, if and only if, the sinner understands that he must behave as one who recognizes that everything is a gift from Hashem. Therefore, the residual money is used to purchase burnt offerings.