Halakhah sa Levitico 27:30: Mishneh Torah, Substitution, Mishneh Torah, Positive Mitzvot & Sefer HaChinukh

One may not engage in shenanigans using an animal [belonging to] the Temple to sanctify its fetus with another [form of] sanctity rather [the fetus] has the sanctity of its mother for the children of sacred animals are sacred from within their mothers' womb as we explained and therefore each of them have the same sanctity as its mother, which cannot be changed in the womb as with a first-born for the first born becomes hole when it comes out. Even though all the laws of the Torah are [arbitrary] decrees, as we explained at the end of the Laws of Desecration, it is worthwhile to meditate on them and assign reasons to whatever part of it you can assign a reason to. Behold the early sages said "Solomon understood the reasons of most of the Torah's Laws." [In light of this] it appears to me that this which the Torah says "and both it and its replacement shall be holy" is similar to the subject where it says "and if the one who sanctifies redeems his house and adds a fifth of your appraisal to it." The Torah descends to the end of a person's train of thought and some of his evil inclination for a person's nature urges him to increase his possessions and conserve his money and even if he pledged and sanctified, he may retract and regret it and redeem it for less than its worth. Therefore if he redeems it privately, the Torah [makes him] add a fifth. Similarly if he sanctified an animal [with] corporeal sanctity he may regret it and since he cannot redeem it, he may come to exchange it for one that is [worth] less and if he were given permission to exchange the worse [animal] for a better one, he would exchange the better animal for a worse one and call it better. Therefore the Scripture closed [the way] in front of him that he may not exchange and fined him if he did exchange and said "both it and its replacement will be holy." And all of these things are to subdue the will and refine the character and most of the laws of the Torah are nothing but advice from the Master of Advice to refine the character and to straighten the actions. Thus it says "Have not I written unto thee excellent things of counsels and knowledge; That I might make thee know the certainty of the words of truth, that thou mightest bring back words of truth to them that send thee?" We have finished the Laws of exchanges with the help of Heaven.

To set apart the tithe of the corn for the Levites, as it is said, “And all the tithe of the land, whether of the seed of the land or of the fruit of the tree, (is the Lord’s)” (Lev. 27:30).

The commandment of the first tithe: That the Children of Israel were obligated to give one-tenth part of the seed of the Land to the Levites, as it is stated (Numbers 18:24), “For it is the tithes of the Children of Israel that they will give to the Lord that I give to the Levites as a gift.” And it is stated in the Order of Eem Bechukotai (Leviticus 27:30), “And all of the tithe of the Land, etc. it is for the Lord.” And that is what is called the first tithe (maaser rishon).