A kindness received should be returned with a freer hand. This is shown by the example of the earth. A passage from Solomon about feasting is adduced to prove the same, and is expounded later in a spiritual sense.
160. It is also right that more regard should be paid to him who has conferred some benefit or gift upon you, if he ever is reduced to want. For what is so contrary to one's duty as not to return what one has received? Nor do I think that a return of equal value should be made, but a greater. One ought to make up for the enjoyment of a kindness one has received from another, to such an extent as to help that person, even to putting an end to his needs. For not to be the better in returning than in conferring a kindness, is to be the inferior; for he who was the first to give was the first in point of time, and also first in showing a kind disposition.
161. Wherefore we must imitate the nature of the earth in this respect, which is wont to return the seed she has received, multiplied a thousand-fold. And so it is written: “As a field is the foolish man, and as a vineyard is the man without sense. If you leave him, he will be made desolate.” As a field also is the wise man, so as to return the seed given him in fuller measure, as though it had been lent to him on interest. The earth either produces fruits of its own accord, or pays back and restores, what it was entrusted with, in fruitful abundance. In both these ways a return is due from you, when you enter upon the use of your father's possession, that you may not be left to lie as an unfruitful field. It may be that a man can make an excuse for not giving anything, but how can he excuse himself for not returning what was given? It is hardly right not to give anything; it is certainly not right to make no return for kindness done to oneself.
162. Therefore Solomon says well: “When you sit to eat at the table of a ruler consider diligently what is before you, and put forth your hand, knowing that it behooves you to make such preparations. But if you are insatiable, be not desirous of his dainties, for they have but a deceptive life.” I have written these words as I wish that we all should follow them. It is a good thing to do a service, but he who knows not how to return one is very hard. The earth herself supplies an example of kindliness. She provides fruits of her own accord, which thou did not sow; she also returns many-fold what she has received. It is not right for you to deny knowledge of money paid in to you, and how can it be right to let a service done go without notice? In the book of Proverbs also it is said: that the repayment of kindness has such great power with God, that through it, even in the day of destruction, a man may find grace, though his sins outweigh all else. And why need I bring forward other examples when the Lord Himself promises in the Gospel a fuller reward to the merits of the saints, and exhorts us to do good works, saying: “Forgive, and you shall be forgiven; give, and it shall be given unto you; good measure, shaken together and running over, shall men give into your bosom.”
163. But the feasting that Solomon speaks of has not to do with common food only, but it is to be understood as having to do with good works. For how can the soul be feasted in better wise than on good works; or what can so easily fill the mind of the just as the knowledge of a good work done? What pleasanter food is there than to do the will of God? The Lord has told us that He had this food alone in abundance, as it is written in the Gospel, saying: “My food is to do the will of My Father which is in heaven.”
164. In this food let us delight of which the prophet says: “Delight thou in the Lord.” In this food they delight, who have with wonderful knowledge learned to take in the higher delights; who can know what that delight is which is pure and which can be understood by the mind. Let us therefore eat the bread of wisdom, and let us be filled with the word of God. For the life of man made in the image of God consists not in bread alone, but in every word that comes from God. About the cup, too, holy Job says, plainly enough: “As the earth waits for the rain, so did they for my words.”
Source: On the Duties of the Clergy (New Advent)