The tithes of Israel were allocated to the Levites as an inheritance and in exchange for their service in the temple:
Numbers 18:21The reasons for which the Levites received tithes play a role in the discussion of whether tithe was limited to agricultural products only. As the above scripture shows, they received tithe for two purposes: as an inheritance and in return for the work of the Tabernacle. If tithe was only required on food items as some claim, then this would mean that the Levites were both given an inheritance and paid for their work in food.
Behold, I have given the children of Levi all the tithes in Israel as an inheritance in return for the work which they perform, the work of the Tabernacle of Meeting.
What inheritance it is talking about, anyway? The inheritance of the other tribes was the land allocated to each family; therefore, receiving the benefit of the land that they would have had makes sense. That's how tithes counted as their inheritance since they would have grown crops and tended herds on their land if they had any.
On the other hand, they were also given tithes as compensation for their work. Anti-tithers are correct in arguing that the economy of the time was not agrarian (i.e., using food as money). In fact, the law states that a person redeem the tithe portion of their crops and give money instead:
Leviticus 27:30-31This fact alone does not prove that tithe was given on a person's monetary income, but it does prove that the Levites must have at least had a system in place for receiving tithes in the form of money - even if only as a result of redemption. The tithing system could not have been merely a food redistribution program for the Levites and the poor! It must have been a more robust system than that, dealing also with money.
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 to the LORD. If a man wants at all to redeem any of his tithes, he shall add one-fifth to it
But back to the point. Surely the Levites received food as an inheritance to replace the land that they were not given. However, did the Levites receive food as payment for their work? I submit that the Levites did not work for peanuts: they received wages for their work in the Tabernacle. Because of redemption, there would always have been tithe in the form of money, and not even the most ardent anti-tither can escape this. In reality though, everyone tithed on their monetary income and the Levites would have gotten quite a lot of tithe in the form of currency.