John Calvin Commentary Deuteronomy 15:19

"All the firstling males that are born of thy herd and of thy flock thou shalt sanctify unto Jehovah thy God: thou shalt do no work with the firstling of thy herd, nor shear the firstling of thy flock." — Deuteronomy 15:19 (ASV)

All the firstborn males. Another caution is added, that they should make no profit from the firstborn; for they might have used the labor of the ox in plowing, or as a beast of burden; they might also have sheared the lambs, and have afterwards brought a deteriorated animal into the tabernacle. God commands, therefore, that what was due to Him should be honestly and absolutely paid.

But if good laws sprang from evil habits, it therefore appears with what audacious greediness men have always been led away to wicked gains, since it was necessary to prohibit them by an express edict from seeking to enrich themselves at God’s expense. Therefore, it is no wonder that men are acute and sagacious in cheating each other, since they by no means hesitate to deceive God by wicked deceptions.