Objection 1. It seems that liberality is not a part of justice. For justice regards that which is due. Now the more a thing is due the less liberally is it given. Therefore liberality is not a part of justice, but is incompatible with it.
Objection 2. Further, justice is about operation as stated above (58, 09; I-II, 60, 2,3): whereas liberality is chiefly about the love and desire of money, which are passions. Therefore liberality seems to belong to temperance rather than to justice.
Objection 3. Further, it belongs chiefly to liberality to give becomingly, as stated (4). But giving becomingly belongs to beneficence and mercy, which pertain to charity, as state above (30,31). Therefore liberality is a part of charity rather than of justice.
On the contrary, Ambrose says (De Offic. i): "Justice has to do with the fellowship of mankind. For the notion of fellowship is divided into two parts, justice and beneficence, also called liberality or kind-heartedness." Therefore liberality pertains to justice.
I answer that, Liberality is not a species of justice, since justice pays another what is his whereas liberality gives another what is one's own. There are, however, two points in which it agrees with justice: first, that it is directed chiefly to another, as justice is; secondly, that it is concerned with external things, and so is justice, albeit under a different aspect, a stated in this Article and above (2, ad 3). Hence it is that liberality is reckoned by some to be a part of justice, being annexed thereto as to a principal virtue.
Reply to Objection 1. Although liberality does not consider the legal due that justice considers, it considers a certain moral due. This due is based on a certain fittingness and not on an obligation: so that it answers to the idea of due in the lowest degree.
Reply to Objection 2. Temperance is about concupiscence in pleasures of the body. But the concupiscence and delight in money is not referable to the body but rather to the soul. Hence liberality does not properly pertain to temperance.
Reply to Objection 3. The giving of beneficence and mercy proceeds from the fact that a man has a certain affection towards the person to whom he gives: wherefore this giving belongs to charity or friendship. But the giving of liberality arises from a person being affected in a certain way towards money, in that he desires it not nor loves it: so that when it is fitting he gives it not only to his friends but also to those whom he knows not. Hence it belong not to charity, but to justice, which is about external things.
Source: Summa Theologica (New Advent)