Adam Clarke on Numbers 18:16

Shalt thou redeem - for the money of five shekels - Redemption of the first-born is one of the rites which is still practiced among the Jews. According to Leo of Modena, it is performed in the following manner: -

When the child is thirty days old, the father sends for one of the descendants of Aaron: several persons being assembled on the occasion, the father brings a cup containing several pieces of gold and silver coin. The priest then takes the child into his arms, and addressing himself to the mother, says: Is this thy son?

Mother. Yes.

Priest. Hast thou never had another child, male or female, a miscarriage or untimely birth?

Mother. No.

Priest. This being the case, this child, as first-born, belongs to me.

Then, turning to the father, he says: If it be thy desire to have this child, thou must redeem it.

Father. I present thee with this gold and silver for this purpose.

Priest. Thou dost wish, therefore, to redeem the child?

Father. I do wish so to do.

The priest then, turning himself to the assembly, says: Very well; this child, as first-born, is mine, as it is written in Bemidbar, (Num 18:16), Thou shalt redeem the first-born of a month old for five shekels, but I shall content myself with this in exchange. He then takes two gold crowns, or thereabouts, and returns the child to his parents.