ייִדישער מחשבֿה אויף ויקרא 27:9

For the same reason animals reserved for holy purposes must not be employed in work; nor is the shearing of such animals permitted (Deut. 15:19). The law concerning the change of a sacrifice must be considered as a preventive; for if it were permitted to substitute a good animal for a bad one, people would substitute a bad animal for a good one, and say that it was better than the original; it was therefore the rule that, if any such change had taken place, both the “original sacrifice and the exchange thereof should be holy” (Lev. 27:9). When a person redeems a thing devoted by him to the Sanctuary, he must likewise add one-fifth (Lev. 27:13, 15); the reason for this is plain. Man is usually selfish, and is naturally inclined to keep and save his property. He would therefore not take the necessary trouble in the interest of the Sanctuary: he would not expose his property sufficiently to the sight of the valuer, and its true value would not be fixed. Therefore the owner had to add one-fifth, whilst a stranger paid only the exact value. These rules were laid down in order that people should not despise that with which the name of God is connected, and which serves as a means of approaching God.