Leviticus 27:30-33

“One-tenth of the produce of the land, whether grain from the fields or fruit from the trees, belongs to the LORD and must be set apart to him as holy. If you want to buy back the LORD’s tenth of the grain or fruit, you must pay its value, plus 20 percent. Count off every tenth animal from your herds and flocks and set them apart for the LORD as holy. You may not pick and choose between good and bad animals, and you may not substitute one for another. But if you do exchange one animal for another, then both the original animal and its substitute will be considered holy and cannot be bought back.”