Since sin is ultimately against God, the one doing wrong must make restitution to the LORD (Leviticus 5:6; Psalm 51:4). God allowed the human victims of the sin to receive this payment. If no victims were alive, the priest would receive the payment on behalf of the LORD. Those doing wrong must also offer a valuable sacrifice to restore their ceremonial purity and their relationship with God (Numbers 5:1–4).
the atonement is made for him: See study note on Leviticus 1:4.