Jamieson, Fausset and Brown on Leviticus 5:15-16

15-16. sin through ignorance, in the holy things of the Lord, &c.--This is a case of sacrilege committed ignorantly, either in not paying the full due of tithes, first-fruits, and similar tribute in eating of meats, which belonged to the priests alone--or he was required, along with the restitution in money, the amount of which was to be determined by the priest, to offer a ram for a trespass offering, as soon as he came to the knowledge of his involuntary fraud.