Objection 1. It would seem lawful to receive money for things annexed to spiritual things. Seemingly all temporal things are annexed to spiritual things, since temporal things ought to be sought for the sake of spiritual things. If, therefore, it is unlawful to sell what is annexed to spiritual things, it will be unlawful to sell anything temporal, and this is clearly false.
Objection 2. Further, nothing would seem to be more annexed to spiritual things than consecrated vessels. Yet it is lawful to sell a chalice for the ransom of prisoners, according to Ambrose (De Offic. ii, 28). Therefore it is lawful to sell things annexed to spiritual things.
Objection 3. Further, things annexed to spiritual things include right of burial, right of patronage, and, according to ancient writers, right of the first-born (because before the Lord the first-born exercised the priestly office), and the right to receive tithes. Now Abraham bought from Ephron a double cave for a burying-place (Genesis 23:8, sqq.), and Jacob bought from Esau the right of the first-born (Genesis 25:31, sqq.). Again the right of patronage is transferred with the property sold, and is granted "in fee." Tithes are granted to certain soldiers, and can be redeemed. Prelates also at times retain for themselves the revenues of prebends of which they have the presentation, although a prebend is something annexed to a spiritual thing. Therefore it is lawful to sell things annexed to spiritual things.
On the contrary, Pope Paschal [Paschal II] says (cf. I, qu. iii, cap. Si quis objecerit): "Whoever sells one of two such things, that the one is unproductive without the other, leaves neither unsold. Wherefore let no person sell a church, or a prebend, or anything ecclesiastical."
I answer that, A thing may be annexed to spiritual things in two ways. First, as being dependent on spiritual things. Thus to have to spiritual things, because it is not competent save to those who hold a clerical office. Hence such things can by no means exist apart from spiritual things. Consequently it is altogether unlawful to sell such things, because the sale thereof implies the sale of things spiritual. Other things are annexed to spiritual things through being directed thereto, for instance the right of patronage, which is directed to the presentation of clerics to ecclesiastical benefices; and sacred vessels, which are directed to the use of the sacraments. Wherefore such things as these do not presuppose spiritual things, but precede them in the order of time. Hence in a way they can be sold, but not as annexed to spiritual things.
Reply to Objection 1. All things temporal are annexed to spiritual things, as to their end, wherefore it is lawful to sell temporal things, but their relation to spiritual things cannot be the matter of a lawful sale.
Reply to Objection 2. Sacred vessels also are annexed to spiritual things as to their end, wherefore their consecration cannot be sold. Yet their material can be sold for the needs of the Church or of the poor provided they first be broken, after prayer has been said over them, since when once broken, they are considered to be no longer sacred vessels but mere metal: so that if like vessels were to be made out of the same material they would have to be consecrated again.
Reply to Objection 3. We have no authority for supposing that the double cave which Abraham bought for a burial place was consecrated for that purpose: wherefore Abraham could lawfully buy that site to be used for burial, in order to turn it into a sepulchre: even so it would be lawful now to buy an ordinary field as a site for a cemetery or even a church. Nevertheless because even among the Gentiles burial places are looked upon as religious, if Ephron intended to accept the price as payment for a burial place, he sinned in selling, though Abraham did not sin in buying, because he intended merely to buy an ordinary plot of ground. Even now, it is lawful in a case of necessity to sell or buy land on which there has previously been a church, as we have also said with regard to sacred vessels (Reply to Objection[2). Or again, Abraham is to be excused because he thus freed himself of a grievance. For although Ephron offered him the burial place for nothing, Abraham deemed that he could not accept it gratis without prejudice to himself.
The right of the first-born was due to Jacob by reason of God's choice, according to Malachi 1:2-3, "I have loved Jacob, but have hated Esau." Wherefore Esau sinned by selling his birthright, yet Jacob sinned not in buying, because he is understood to have freed himself of his grievance.
The right of patronage cannot be the matter of a direct sale, nor can it be granted "in fee," but is transferred with the property sold or granted.
The spiritual right of receiving tithes is not granted to layfolk, but merely the temporal commodities which are granted in the name of tithe, as stated above (Question 87, Article 3).
With regard to the granting of benefices it must, however, be observed, that it is not unlawful for a bishop, before presenting a person to a benefice, to decide, for some reason, to retain part of the revenues of the benefice in question, and to spend it on some pious object. But, on the other hand, if he were to require part of the revenues of that benefice to be given to him by the beneficiary, it would be the same as though he demanded payment from him, and he would not escape the guilt of simony.
Source: Summa Theologica (New Advent)