Redemption of the Son in Judea

You explained that the basis of a law is the right of another person, and the basis of a law is my duty. Therefore, charity, interest, redemption of the son, and more appear in the basis of a law and not in the context of a law.
And here, the Harashak in Shaarei Yosher 52 writes something like this: "Likewise, it is an obligation, and the encumbrance of money is a legal obligation that is imposed according to the ways of the owners, or that is imposed by the Torah , such as damages and redemption of a son, and the like."
And how/why does the author include the law of the redemption of the son within the legal laws even though it probably (at least according to my intuition, and in light of the fact that it appears in the Jewish Law) belongs to the religious-ritual realm?