MERCIMONIO. – In general, this denotes any illicit traffic, whatever the source of its unlawfulness. In a narrower sense, m. denotes illicit traffic because it is conducted in forbidden goods; either because, by their intrinsic dignity, they are incompatible with the condition of a marketable commodity, or because the law so provides in order to protect the public or private good.
Every contract concerning a forbidden object is null; consequently, the reciprocal obligations that the contracting parties freely believed they were undertaking do not exist. However, after fulfilling them, the parties may, in conscience, retain the benefits received, without any obligation of restitution. This is not by virtue of the preceding contract, which, because of its invalidity, always remains ineffective; rather, it is by virtue of the very fulfillment of the contractual promises. Such fulfillment, in itself neither lawful nor obligatory, nevertheless verifies the condition on which the other innominate contract «do ut des» depended, which would always be implicit in any illicit transaction. By virtue of this atypical contract, the party who in fact promptly performs the sinful act agreed upon is owed the counterperformance by the other party, provided that it does not consist in an intrinsically illicit act; e.g., the agreed price.
It is clear that the concrete forms of m. may be very numerous. Within the sphere of moral theology, however, the characteristic term principally calls to mind two: prostitution and the m. of Masses. The evident dignity of the matter bartered in both forms of m. stigmatizes them both. Even if positive law, considering that it thereby avoids a greater evil to social life, grants tolerance to prostitution, it does not thereby make it morally lawful either for those who practice it or for those who resort to II. Nevertheless, according to the general doctrine set out above, the price of infamy may calmly be retained in conscience. This is also confirmed by a decree of the S. Penitentiary of 23 April 1822. The other aspects of this sad phenomenon are not relevant here (v. PROSTITUZIONE and the bibliography cited there).
For the m. of Masses, V. ELEMOSINE.