John Calvin Commentary Leviticus 27:30

"And all the tithe of the land, whether of the seed of the land, or of the fruit of the tree, is Jehovah`s: it is holy unto Jehovah." — Leviticus 27:30 (ASV)

And all the tithe of the land. In these words, God shows that in assigning the tithes to the Levites, He ceded His own rights, because they were a kind of royal revenue. Thus, He prevents all complaint, since otherwise the other tribes might have grumbled about being unjustly burdened. He therefore appoints the priests as His receivers, to collect in His name what could not be refused without impious and sacrilegious fraud.

Regarding the provision that a fifth part should be added to the value when tithes are redeemed by a money payment, the object is not for the Levites to profit from others' losses. Instead, because property owners craftily sought some advantage in this exchange of grain for money, this rule prevents frauds by which the Levites would lose something through such deceptive exchanges. On the same grounds, He commands that the animals, whatever they might be, should be given as tithe and does not permit them to be redeemed by money. This is because, if the choice had been free, no fat or healthy animal would have ever come to the Levites.

Therefore, this law provided a remedy for avarice and stinginess, and with good reason. For if the proverb is true that “good laws spring from evil habits,”²¹⁶ it was necessary that such a covetous and ill-disposed people should be restrained in the path of duty by the greatest severity. And although such careful provision was made for the Levites, there was scarcely any period in which they did not suffer from want, and sometimes they wandered about half-starved. Indeed, after the return from the Babylonian captivity, the memory of so great a blessing did not prevent a part of the tithes from being secretly withheld from them, as God complains in Malachi 3:8. From this, it appears that it was not without purpose that the people were so strictly commanded to pay them.

²¹⁶ See Tacitus Ann. 15:20. “See Tacitus Ann. 15:20. “Usu probatum est, patres conseripti, leges egregias, exempla honesta, apud bonos ex delictis aliorum gigni, etc."."