18 “Bringing forth grass for the cattle, and green herb for the service of men”. This is true, I perceive; I recognise the creation: the earth does bring forth grass for the cattle, and green herb for the service of men. But I perceive the words, “You shall not muzzle the mouth of the ox which treads out the grain: Does God take care for oxen? For our sakes therefore the Scripture says it.” How then does the earth bring forth grass for the cattle? Because “the Lord has ordained that they which preach the Gospel should live of the Gospel.”
He sent preachers, saying unto them, “Eat such things as are set before you of them: for the labourer is worthy of his hire.”...They give spiritual, they receive carnal things; they give gold, they receive grass....“If we have sown unto you spiritual things, is it a great matter if we shall reap your carnal things?” This the Apostle said, a preacher so laborious, so indefatigable, so well tried, that he gives this very grass to the earth. “Nevertheless,” he says, “we have not used this power.”
He shows that it is due to him, yet he received it not; nor has he condemned those who have received what was due. For those were to be condemned who exact what is not due, not they who accept their recompense: yet he gave up even his own recompense. You do not cease to owe to another, because one has given up his dues, otherwise you will not be the watered earth which brings forth grass for the cattle.... You receive spiritual things, give carnal things in return: to the soldier they are due, to the soldier you return them; you are the paymaster of Christ.
“Who goes a warfare any time at his own charges? Who plants a vineyard, and eats not of the fruit thereof? Or who feeds a flock, and eats not of the milk of the flock? I speak not thus, that it should be so done unto me.” There has been such a soldier as gave up his rations of food even to the paymaster: yet let the paymaster pay the rations....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)