Charles Ellicott Commentary Leviticus 5:18

"And he shall bring a ram without blemish out of the flock, according to thy estimation, for a trespass-offering, unto the priest; and the priest shall make atonement for him concerning the thing wherein he erred unwittingly and knew it not, and he shall be forgiven." — Leviticus 5:18 (ASV)

And he shall bring a ram. —Under such circumstances of suspense and feelings of guilt, he is to bring the same victim as in the former instance.

With your estimation. —That is, according to your i.e., Moses’ valuation, the ram is to be worth two shekels. (See Leviticus 5:15.)

And knew it not. —Better, though he wist or knew not, the precise sacred thing which he used, as the same phrase is rendered in the preceding verse.

That is, to be on the right side, the priest shall make an atonement for him concerning this error of inadvertence, though the offender is uncertain whether he actually committed the offence or not. Still, as the case is a doubtful one, he is exempt from the additional fifth part which the transgressor had to pay who indisputably committed this offence in ignorance. (See Leviticus 5:16.)