Calvin's Commentaries, Vol. 4: Harmony of the Law, Part II, tr. by John King, [1847-50], at sacred-texts.com
30. 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: it is holy unto the LORD.
30. Onmes autem decimae terrae sive de semine terrae, sive de fructu arborum, Jehovae sunt, sanctitas Jehovae.
31. And if a man will at all redeem ought of his tithes, he shall add thereto the fifth part thereof.
31. Si vero redimendo redemerit vir allquid de decimis suis, quintam ejus partem addet ultra eam.
32. And concerning the tithe of the herd, or of the flock, even of whatsoever passeth under the rod, the tenth shall be holy unto the LORD.
32. Et onmis decima boum et ovium, omnium nempe quae transeunt sub virga, decima inquam pars erit sanctitas Jehovae.
33. He shall not search whether it be good or bad, neither shall he change it: and if he change it at all, then both it and the change thereof shall be holy; it shall not be redeemed.
33. Non disquiret utrum bonum an malum, neque permutabit illud: quod si commutando commutaverit ipsum, erit tam hoc quam illud quod commutatum est, erit inquam sanctificatio, non redimetur.
30. And all the tithe of the land. In these words God shews that in assigning the tithes to the Levites, He ceded His own rights, inasmuch as they were a kind of royal revenue; and thus He bars all complaint, since otherwise the other tribes might have murmured on being unduly burdened. He therefore appoints the priests as His receivers, to collect in His name what could not be refused without impious and sacrilegious fraudulency. In the provision that, where the tithes are redeemed by a money payment, a fifth part should be added to their value, the object is not that the Levites should make a gain of the loss of others; but, because the owners of property craftily aimed at some advantage in this commutation of corn for money, frauds are thus prevented whereby something would be lost to the Levites by this deceptive exchange. 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, since, if the choice had been free, no fat or healthy animal would have ever come to the Levites. Therefore, in this law a remedy was applied to avarice and meanness, and not without good cause; for if the proverb be true, that “good laws spring from evil habits,” 216 it was necessary that so covetous and ill-disposed a people should be restrained in the path of duty by the utmost severity. And although such careful provision was made for the Levites, yet there was scarcely any period in which they did not suffer from want, and sometimes they wandered about half-starved; nay, after the return from the Babylonish captivity, the memory of so great a blessing did not prevent a part of the tithes from being surreptitiously withheld from them; as God complains in Mal 3:8. Whence it appears that it was not without purpose that the people were so imperiously enjoined to pay them.
See Tacitus Ann. 15:20. “Usu probatum est, patres conseripti, leges egregias, exempla honesta, apud bonos ex delictis aliorum gigni, etc."