Alexander Gray writes about St. Thomas' economics:
Two rights... must be distinguished in this matter. There is the power to acquire and administer; there is the power to use.
Now, so far as concerns acquisition and administration, St. Thomas largely follows Aristotle, arguing that private ownership is not merely permissible, but necessary to human life.
While private ownership is thus consecrated on the side of administration, it is otherwise so far as concerns 'use.'
... the principle of community in use, of making wealth serviceable to the community at large, could only be effected on the initiative of the possessor of wealth, through the voluntary exercise of alms-giving.
The possessor of wealth is the administrator merely; it may be that in his administration he is responsible solely to his own judgment; but he holds it only on condition that he uses it for the good of mankind, which involves the giving of alms. (Socialist Tradition, 56ff)
But then accumulation of capital and growing one's business fall under "acquisition and administration," which is fully sufficient to justify private property in the means of production.
Further, now St. Thomas would realize that businessmen compete to satisfy consumers increasingly better with time; it isn't really true that "the rich man, if he does not give alms, is a thief."
In his time there was not a vast layer of bourgeoisie / middle class. Alms-giving would then fall on every non-pauper, i.e., on the vast majority of the population in a capitalist society. Nowhere does St. Thomas favor "redistribution" or taxation or "welfare state."
Nor would he be able to define "superfluity" non-arbitrarily, for the familiar reason that today's luxuries are tomorrow's necessities.
Thus, I expect that St. Thomas, were he living now, would find privately invested capital fine and dandy, but insist that when one sells his stocks or takes dividends for personal consumption, he meets a general and imperfect duty of alms-giving.
Another interesting point made by Gray is that St. Thomas
belonged to an age which believed that men were assigned by Providence to different stations in life and, as has been seen, that it was their first duty to live in accordance with the requirements of that station.
He was therefore no egalitarian.
He outlined a theory of division of labor by divine decree, according to which one is more drawn to one task than another. (59)
Indeed, St. Thomas argues that giving alms
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. (ST, II-II, 32, 6)
He was no Cohenian socialist who keeps the appearance of the free market but demands egalitarianism in consumer goods.