"You are not making a gift of what is yours to the poor man, but you are giving him back what is his. You have been appropriating things that are meant to be for the common use of everyone. The earth belongs to everyone, not to the rich."
A bit of research led me to a number of mentions of this passage, including Pope Paul VI's 1967 encyclical, Populorum Progressio, which sets it in this context:
Issues and Principles
22. In the very first pages of Scripture we read these words: "Fill the earth and subdue it."(19) This teaches us that the whole of creation is for man, that he has been charged to give it meaning by his intelligent activity, to complete and perfect it by his own efforts and to his own advantage.
Now if the earth truly was created to provide man with the necessities of life and the tools for his own progress, it follows that every man has the right to glean what he needs from the earth. The recent Council reiterated this truth: "God intended the earth and everything in it for the use of all human beings and peoples. Thus, under the leadership of justice and in the company of charity, created goods should flow fairly to all." (20)
All other rights, whatever they may be, including the rights of property and free trade, are to be subordinated to this principle. They should in no way hinder it; in fact, they should actively facilitate its implementation. Redirecting these rights back to their original purpose must be regarded as an important and urgent social duty.
The Use of Private Property
23. "He who has the goods of this world and sees his brother in need and closes his heart to him, how does the love of God abide in him?" (21) Everyone knows that the Fathers of the Church laid down the duty of the rich toward the poor in no uncertain terms. As St. Ambrose put it: "You are not making a gift of what is yours to the poor man, but you are giving him back what is his. You have been appropriating things that are meant to be for the common use of everyone. The earth belongs to everyone, not to the rich." (22) These words indicate that the right to private property is not absolute and unconditional.
No one may appropriate surplus goods solely for his own private use when others lack the bare necessities of life. In short, "as the Fathers of the Church and other eminent theologians tell us, the right of private property may never be exercised to the detriment of the common good." When "private gain and basic community needs conflict with one another," it is for the public authorities "to seek a solution to these questions, with the active involvement of individual citizens and social groups." (23)
The Common Good
24. If certain landed estates impede the general prosperity because they are extensive, unused or poorly used, or because they bring hardship to peoples or are detrimental to the interests of the country, the common good sometimes demands their expropriation.
Vatican II affirms this emphatically. (24) At the same time it clearly teaches that income thus derived is not for man's capricious use, and that the exclusive pursuit of personal gain is prohibited. Consequently, it is not permissible for citizens who have garnered sizeable income from the resources and activities of their own nation to deposit a large portion of their income in foreign countries for the sake of their own private gain alone, taking no account of their country's interests; in doing this, they clearly wrong their country. (25)
(19) Gn 1. 28.
(20) Church in the World of Today, no. 69: AAS 58 (1966), 1090 [cf. TPS XI, 306].
(21) 1 Jn 3. 17.
(22) De Nabute, c. 12, n. 53: PL 14. 747; cf. J. R. Palanque, Saint Ambroise et l'empire romain,Paris: de Boccard (1933), 336 ff.
(23) Letter to the 52nd Social Week at Brest, in L'homme et la révolution urbaine, Lyon: Chronique sociale (1965), 8-9.
(24) Church in the World of Today, no. 71: AAS 58 (1966), 1093 [cf. TPS XI, 308].
(25) Ibid., no. 65: AAS 58 (1966), 1086 [cf. TPS XI, 303].
Those interested in Catholic Social Thought should look for a new book which came out of a 2007 conference held at the University of Scranton, and edited by Professor Kenneth R. Lord of UScranton, entitled "Two Views of Social Justice: A Catholic/Georgist Dialogue." The version I've seen is the October, 2012, issue of The American Journal of Economics and Sociology, and I understand that it will be made available in other forms as well.
The abstract for the book:
Sixteen scholars have come together in this issue to examine eight social-justice themses from the perspectives of Catholic Social Thought and the philosophy of Henry George. The themes they address are natural law, human nature, the nature of work, the nineteenth-century papal encyclical Rerum Novarum, causes of war, immigration, development, and wealth, and neighborhood revitalization. While they sometimes wrangle with each other, their common aspiration is the same as their nineteenth-century predecessors,: to find solutions to the human suffering caused by injustice.