Objection 1. It would seem that one may give alms out of ill-gotten goods. For it is written (Luke 16:9): "Make unto you friends of the mammon of iniquity." Now mammon signifies riches. Therefore it is lawful to make unto oneself spiritual friends by giving alms out of ill-gotten riches.
Objection 2. Further, all filthy lucre seems to be ill-gotten. But the profits from whoredom are filthy lucre; wherefore it was forbidden (Deuteronomy 23:18) to offer therefrom sacrifices or oblations to God: "Thou shalt not offer the hire of a strumpet... in the house of... thy God." On like manner gains from games of chance are ill-gotten, for, as the Philosopher says (Ethic. iv, 1), "we take such like gains from our friends to whom we ought rather to give." And most of all are the profits from simony ill-gotten, since thereby the Holy Ghost is wronged. Nevertheless out of such gains it is lawful to give alms. Therefore one may give alms out of ill-gotten goods.
Objection 3. Further, greater evils should be avoided more than lesser evils. Now it is less sinful to keep back another's property than to commit murder, of which a man is guilty if he fails to succor one who is in extreme need, as appears from the words of Ambrose who says (Cf. Canon Pasce dist. lxxxvi, whence the words, as quoted, are taken): "Feed him that dies of hunger, if thou hast not fed him, thou hast slain him". Therefore, in certain cases, it is lawful to give alms of ill-gotten goods.
On the contrary, Augustine says (De Verb. Dom. xxxv, 2): "Give alms from your just labors. For you will not bribe Christ your judge, not to hear you with the poor whom you rob... Give not alms from interest and usury: I speak to the faithful to whom we dispense the Body of Christ."
I answer that, A thing may be ill-gotten in three ways. On the first place a thing is ill-gotten if it be due to the person from whom it is gotten, and may not be kept by the person who has obtained possession of it; as in the case of rapine, theft and usury, and of such things a man may not give alms since he is bound to restore them.
Secondly, a thing is ill-gotten, when he that has it may not keep it, and yet he may not return it to the person from whom he received it, because he received it unjustly, while the latter gave it unjustly. This happens in simony, wherein both giver and receiver contravene the justice of the Divine Law, so that restitution is to be made not to the giver, but by giving alms. The same applies to all similar cases of illegal giving and receiving.
Thirdly, a thing is ill-gotten, not because the taking was unlawful, but because it is the outcome of something unlawful, as in the case of a woman's profits from whoredom. This is filthy lucre properly so called, because the practice of whoredom is filthy and against the Law of God, yet the woman does not act unjustly or unlawfully in taking the money. Consequently it is lawful to keep and to give in alms what is thus acquired by an unlawful action.
Reply to Objection 1. As Augustine says (De Verb. Dom. 2), "Some have misunderstood this saying of Our Lord, so as to take another's property and give thereof to the poor, thinking that they are fulfilling the commandment by so doing. This interpretation must be amended. Yet all riches are called riches of iniquity, as stated in De Quaest. Ev. ii, 34, because "riches are not unjust save for those who are themselves unjust, and put all their trust in them. Or, according to Ambrose in his commentary on Luke 16:9, "Make unto yourselves friends," etc., "He calls mammon unjust, because it draws our affections by the various allurements of wealth." Or, because "among the many ancestors whose property you inherit, there is one who took the property of others unjustly, although you know nothing about it," as Basil says in a homily (Hom. super Luc. A, 5). Or, all riches are styled riches "of iniquity," i.e., of "inequality," because they are not distributed equally among all, one being in need, and another in affluence.
Reply to Objection 2. We have already explained how alms may be given out of the profits of whoredom. Yet sacrifices and oblations were not made therefrom at the altar, both on account of the scandal, and through reverence for sacred things. It is also lawful to give alms out of the profits of simony, because they are not due to him who paid, indeed he deserves to lose them. But as to the profits from games of chance, there would seem to be something unlawful as being contrary to the Divine Law, when a man wins from one who cannot alienate his property, such as minors, lunatics and so forth, or when a man, with the desire of making money out of another man, entices him to play, and wins from him by cheating. On these cases he is bound to restitution, and consequently cannot give away his gains in alms. Then again there would seem to be something unlawful as being against the positive civil law, which altogether forbids any such profits. Since, however, a civil law does not bind all, but only those who are subject to that law, and moreover may be abrogated through desuetude, it follows that all such as are bound by these laws are bound to make restitution of such gains, unless perchance the contrary custom prevail, or unless a man win from one who enticed him to play, in which case he is not bound to restitution, because the loser does not deserve to be paid back: and yet he cannot lawfully keep what he has won, so long as that positive law is in force, wherefore in this case he ought to give it away in alms.
Reply to Objection 3. All things are common property in a case of extreme necessity. Hence one who is in such dire straits may take another's goods in order to succor himself, if he can find no one who is willing to give him something. For the same reason a man may retain what belongs to another, and give alms thereof; or even take something if there be no other way of succoring the one who is in need. If however this be possible without danger, he must ask the owner's consent, and then succor the poor man who is in extreme necessity.
Source: Summa Theologica (New Advent)