Charles Ellicott Commentary Leviticus 5:16

"and he shall make restitution for that which he hath done amiss in the holy thing, and shall add the fifth part thereto, and give it unto the priest; and the priest shall make atonement for him with the ram of the trespass-offering, and he shall be forgiven." — Leviticus 5:16 (ASV)

And he shall make amends. — Since the sacrifice was simply to atone for the transgression, the offender was first to make restitution of the full value of the principal which he had inadvertently appropriated.

And shall add the fifth part thereto. — Besides paying the principal, the fifth part of the value of the holy property thus restored is to be added to the original amount. According to the rules that prevailed in the time of Christ, the principal was regarded as four-fifths of the total sum paid, and the remaining one-fifth was the part added. For instance, if the offender had consumed holy things to the value of four shekels, he had to pay five shekels, this fifth part being added to the four. According to our method of calculation, this added amount is one-fourth. No distinction is made here whether the offender is the high priest, a prince, or a private individual.