Objection 1. It would seem that a man is bound to restore what he has not taken. For he that has inflicted a loss on a man is bound to remove that loss. Now it happens sometimes that the loss sustained is greater than the thing taken: for instance, if you dig up a man's seeds, you inflict on the sower a loss equal to the coming harvest, and thus you would seem to be bound to make restitution accordingly. Therefore a man is bound to restore what he has not taken.
Objection 2. Further, he who retains his creditor's money beyond the stated time, would seem to occasion his loss of all his possible profits from that money, and yet he does not really take them. Therefore it seems that a man is bound to restore what he did not take.
Objection 3. Further, human justice is derived from Divine justice. Now a man is bound to restore to God more than he has received from Him, according to Matthew 25:26, "Thou knewest that I reap where I sow not, and gather where I have not strewed." Therefore it is just that one should restore to a man also, something that one has not taken.
On the contrary, Restitution belongs to justice, because it re-establishes equality. But if one were to restore what one did not take, there would not be equality. Therefore it is not just to make such a restitution.
I answer that, Whoever brings a loss upon another person, seemingly, takes from him the amount of the loss, since, according to the Philosopher (Ethic. v, 4) loss is so called from a man having "less"* than his due. [The derivation is more apparent in English than in Latin, where 'damnum' stands for 'loss,' and 'minus' for 'less.' Aristotle merely says that to have more than your own is called 'gain,' and to have less than you started with is called 'loss.'] Therefore a man is bound to make restitution according to the loss he has brought upon another.
Now a man suffers a loss in two ways. First, by being deprived of what he actually has; and a loss of this kind is always to be made good by repayment in equivalent: for instance if a man damnifies another by destroying his house he is bound to pay him the value of the house. Secondly, a man may damnify another by preventing him from obtaining what he was on the way to obtain. A loss of this kind need not be made good in equivalent; because to have a thing virtually is less than to have it actually, and to be on the way to obtain a thing is to have it merely virtually or potentially, and so were he to be indemnified by receiving the thing actually, he would be paid, not the exact value taken from him, but more, and this is not necessary for salvation, as stated above. However he is bound to make some compensation, according to the condition of persons and things.
From this we see how to answer the First and Second Objections: because the sower of the seed in the field, has the harvest, not actually but only virtually. On like manner he that has money has the profit not yet actually but only virtually: and both may be hindered in many ways.
Reply to Objection 3. God requires nothing from us but what He Himself has sown in us. Hence this saying is to be understood as expressing either the shameful thought of the lazy servant, who deemed that he had received nothing from the other, or the fact that God expects from us the fruit of His gifts, which fruit is from Him and from us, although the gifts themselves are from God without us.
Source: Summa Theologica (New Advent)