Objection 1. It seems that prodigality is not opposite to covetousness. For opposites cannot be together in the same subject. But some are at the same time prodigal and covetous. Therefore prodigality is not opposite to covetousness.
Objection 2. Further, opposites relate to one same thing. But covetousness, as opposed to liberality, relates to certain passions whereby man is affected towards money: whereas prodigality does not seem to relate to any passions of the soul, since it is not affected towards money, or to anything else of the kind. Therefore prodigality is not opposite to covetousness.
Objection 3. Further, sin takes its species chiefly from its end, as stated above (I-II, 62, 3). Now prodigality seems always to be directed to some unlawful end, for the sake of which the prodigal squanders his goods. Especially is it directed to pleasures, wherefore it is stated (Luke 15:13) of the prodigal son that he "wasted his substance living riotously." Therefore it seems that prodigality is opposed to temperance and insensibility rather than to covetousness and liberality.
On the contrary, The Philosopher says (Ethic. ii, 7; iv, 1) that prodigality is opposed to liberality, and illiberality, to which we give here the name of covetousness.
I answer that, In morals vices are opposed to one another and to virtue in respect of excess and deficiency. Now covetousness and prodigality differ variously in respect of excess and deficiency. Thus, as regards affection for riches, the covetous man exceeds by loving them more than he ought, while the prodigal is deficient, by being less careful of them than he ought: and as regards external action, prodigality implies excess in giving, but deficiency in retaining and acquiring, while covetousness, on the contrary, denotes deficiency in giving, but excess in acquiring and retaining. Hence it is evident that prodigality is opposed to covetousness.
Reply to Objection 1. Nothing prevents opposites from being in the same subject in different respects. For a thing is denominated more from what is in it principally. Now just as in liberality, which observes the mean, the principal thing is giving, to which receiving and retaining are subordinate, so, too, covetousness and prodigality regard principally giving. Wherefore he who exceeds in giving is said to be "prodigal," while he who is deficient in giving is said to be "covetous." Now it happens sometimes that a man is deficient in giving, without exceeding in receiving, as the Philosopher observes (Ethic. iv, 1). And in like manner it happens sometimes that a man exceeds in giving, and therefore is prodigal, and yet at the same time exceeds in receiving. This may be due either to some kind of necessity, since while exceeding in giving he is lacking in goods of his own, so that he is driven to acquire unduly, and this pertains to covetousness; or it may be due to inordinateness of the mind, for he gives not for a good purpose, but, as though despising virtue, cares not whence or how he receives. Wherefore he is prodigal and covetous in different respects.
Reply to Objection 2. Prodigality regards passions in respect of money, not as exceeding, but as deficient in them.
Reply to Objection 3. The prodigal does not always exceed in giving for the sake of pleasures which are the matter of temperance, but sometimes through being so disposed as not to care about riches, and sometimes on account of something else. More frequently, however, he inclines to intemperance, both because through spending too much on other things he becomes fearless of spending on objects of pleasure, to which the concupiscence of the flesh is more prone; and because through taking no pleasure in virtuous goods, he seeks for himself pleasures of the body. Hence the Philosopher says (Ethic. iv, 1) "that many a prodigal ends in becoming intemperate."
Source: Summa Theologica (New Advent)