Objection 1. It would seem that one ought not to give alms out of what one needs. For the order of charity should be observed not only as regards the effect of our benefactions but also as regards our interior affections. Now it is a sin to contravene the order of charity, because this order is a matter of precept. Since, then, the order of charity requires that a man should love himself more than his neighbor, it seems that he would sin if he deprived himself of what he needed, in order to succor his neighbor.
Objection 2. Further, whoever gives away what he needs himself, squanders his own substance, and that is to be a prodigal, according to the Philosopher (Ethic. iv, 1). But no sinful deed should be done. Therefore we should not give alms out of what we need.
Objection 3. Further, the Apostle says (1 Timothy 5:8): "If any man have not care of his own, and especially of those of his house, he hath denied the faith, and is worse than an infidel." Now if a man gives of what he needs for himself or for his charge, he seems to detract from the care he should have for himself or his charge. Therefore it seems that whoever gives alms from what he needs, sins gravely.
On the contrary, Our Lord said (Matthew 19:21): "If thou wilt be perfect, go, sell what thou hast, and give to the poor." Now he that gives all he has to the poor, gives not only what he needs not, but also what he needs. Therefore a man may give alms out of what he needs.
I answer that, A thing is necessary in two ways: first, because without it something is impossible, and it is altogether wrong to give alms out of what is necessary to us in this sense; for instance, if a man found himself in the presence of a case of urgency, and had merely sufficient to support himself and his children, or others under his charge, he would be throwing away his life and that of others if he were to give away in alms, what was then necessary to him. Yet I say this without prejudice to such a case as might happen, supposing that by depriving himself of necessaries a man might help a great personage, and a support of the Church or State, since it would be a praiseworthy act to endanger one's life and the lives of those who are under our charge for the delivery of such a person, since the common good is to be preferred to one's own.
Secondly, a thing is said to be necessary, if a man cannot without it live in keeping with his social station, as regards either himself or those of whom he has charge. The "necessary" considered thus is not an invariable quantity, for one might add much more to a man's property, and yet not go beyond what he needs in this way, or one might take much from him, and he would still have sufficient for the decencies of life in keeping with his own position. Accordingly it is good to give alms of this kind of "necessary"; and it is a matter not of precept but of counsel. Yet it would be inordinate to deprive oneself of one's own, in order to give to others to such an extent that the residue would be insufficient for one to live in keeping with one's station and the ordinary occurrences of life: for no man ought to live unbecomingly. There are, however, three exceptions to the above rule. The first is when a man changes his state of life, for instance, by entering religion, for then he gives away all his possessions for Christ's sake, and does the deed of perfection by transferring himself to another state. Secondly, when that which he deprives himself of, though it be required for the decencies of life, can nevertheless easily be recovered, so that he does not suffer extreme inconvenience. Thirdly, when he is in presence of extreme indigence in an individual, or great need on the part of the common weal. For in such cases it would seem praiseworthy to forego the requirements of one's station, in order to provide for a greater need.
The objections may be easily solved from what has been said.
Source: Summa Theologica (New Advent)