THE CODIFIED LAW (The Written Law of Moses)
Tithing Instituted as Part of the Covenant (Leviticus 27:30-34)
30″ ‘A tithe of everything from the land, whether grain from the soil or fruit from the trees, belongs to the Lord; it is holy to the Lord. 31If a man redeems any of his tithe, he must add a fifth of the value to it. 32The entire tithe of the herd and flock—every tenth animal that passes under the shepherd’s rod—will be holy to the Lord. 33He must not pick out the good from the bad or make any substitution. If he does make a substitution, both the animal and its substitute become holy and cannot be redeemed.’ ” 34These are the commands the Lord gave Moses on Mount Sinai for the Israelites.
Under the law, one tenth of everything from the land—plant or animal—was considered holy and belonged to God. This portion could be redeemed, but a fifth part of the value had to be added. There could be no substitutions or alterations (switch one cow for another, for instance; if you tried and someone found out, both would be considered part of the tithe). It wasn’t until Numbers 18 (following) that further instruction was given in regard to implementing the tithe; what to do with the cows, grain, etc.