Numbers 18:24 But the tithes of the children of Israel, which they offer as an heave offering unto the LORD, I have given to the Levites to inherit: therefore I have said unto them, Among the children of Israel they shall have no inheritance.
The Levites had no other income than the offerings. They were to live of the tithes. That was God’s way of taking care of His people the Levites in Israel.
But the Levites were to tithe of the tithe that they had received
Numbers 18:26 Thus speak unto the Levites, and say unto them, When ye take of the children of Israel the tithes which I have given you from them for your inheritance, then ye shall offer up an heave offering of it for the LORD, even a tenth part of the tithe.
The Levites tithed to the Lord just like the regular people. God said that He consider their service in the tabernacle and their service to Him as their work. When they gave a tenth of what they had received it was just like the other guy who gave a tithe of his own work.
Numbers 18:27 And this your heave offering shall be reckoned unto you, as though it were the corn of the threshingfloor, and as the fulness of the winepress. 30 Therefore thou shalt say unto them, When ye have heaved the best thereof from it, then it shall be counted unto the Levites as the increase of the threshingfloor, and as the increase of the winepress.
No man of God should have ever thought that they have to give but we are freed from giving. That wasn’t the teaching at all. Actually they were to all tithe in acknowledgement of the fact that it was God giving them all that they had.