(b) Oaths added to contracts have moral effects on the contracts themselves and also on acts contrary to them. As regards the contract, an oath adds the obligation of religion to that of justice, if the contract is valid and irrescindable; and the common opinion is that it strengthens a contract extrinsically, that is, it induces an obligation of religion to keep the promise, if the oath is invalid or rescindable by positive law only and in favor of a private privilege; but an oath in no way strengthens a contract that is naturally invalid or rescindable positively on account of the public good. As regards acts that are contrary to an invalid or rescindable contract that was confirmed by a valid oath, they are sinful, as being irreligious, but not invalid nor unjust (see 2260).
(c) Conditions are accidents or circumstances so added to a contract that the consent or dissent is made dependent upon their existence or fulfillment. An immoral condition, if unfulfilled, takes from the contract all obligation, exception being made for separable parts that are not affected by the immoral clause; but if it has been fulfilled, it seems that there is a moral obligation to pay the consideration promised (see 1878 d).
(d) Modes are accidents or circumstances so added to a contract as to qualify the rights or duties of the contractants, or the purpose, matter or time of the contract, but not so as to make the consent dependent on the fulfillment of the thing designated. Thus, if Titus leaves money to Balbus, chiefly because Balbus is his nephew, and secondarily because he imposes on Balbus the obligation of using the money for his education, Balbus in accepting the money accepts also the obligation, but the gift does not lapse if the obligation is not complied with. If a donor adds an immoral mode to his gift (e.g., that the donee use in immoral ways the money left him), this purpose is regarded as non-existent and the gift stands in spite of it. If an agent violates a mode (e.g., he pays $1001 when he was directed to pay $1000) but not a condition (e.g., that he purchase land and not a house), the contract stands.
1887. The Moral Obligation of Entering into a Contract.—(a) There is a duty of justice when one is under public or private engagement to make a contract. Examples are a merchant who opens a store for public patronage, or an auctioneer who holds a sale before invited patrons, or an owner who makes with another person a contract to sell, or a man and woman who make solemn espousals.
(b) There is a duty of charity when a neighbor is in such need that he deserves to be helped, for example, by a loan or by assistance to make a loan: “From him that would borrow of thee turn not away” (Matt., v. 42); “A good man is surety for his neighbor” (Ecclus., xxix. 18).
1888. Every valid contract obliges to faithful performance as a duty of conscience, even though it be unenforceable and without civil obligation. We shall discuss the properties of this obligation.
(a) Quality of the Obligation.—Onerous contracts oblige in virtue of commutative justice and under pain of restitution; gratuitous contracts oblige according to some from justice, according to others from fidelity, according to others from fidelity or justice as the obligor intends (see 1692, 1753). In practice one may follow the rule that a liberal promise or wager or other gratuitous contract obliges only from fidelity with no duty of restitution, unless it be certain that the promisor intended to bind himself in justice. One is responsible, however, for damages resultant on breach of promise. The obligation seems to be one of legal justice only when the thing promised is something on which a pecuniary value cannot be set and consists in compliance with law (e.g., in suretyship or bail for keeping the peace or appearing in court).
(b) Quantity of Obligation.—In onerous contracts the degree of obligation depends on the importance of the subject matter, and hence it is a mortal sin to violate a contract in which a grave right is concerned; in gratuitous contracts the degree of obligation depends entirely, according to some, on the will of the person who liberally binds himself, but others hold that it depends on the importance of the subject-matter.
(c) Subjects of Obligation.—The parties to the contracts and those who take their place (e.g., heirs, executors) or who are responsible for the contract (e.g., those who commanded the agreement) are morally bound to fulfill the agreement, while others are bound not to interfere with the fulfillment.
(d) Objects of Obligation.—Directly, there is the duty of observing what is contained explicitly or implicitly in the agreement, and indirectly of making good any losses caused by breach of contract. A rescindable contract obliges until it is lawfully disaffirmed by the party who has the right to break it; a quasi-contract imposes on the party who has benefited by the services or expenses of another a moral obligation of making compensation. If a contract transfers ownership (e.g., contract of sale passing title to buyer, _mutuum_), the transferee must bear the risks and expenses of the thing transferred; but if it does not transfer ownership (e.g., contract to sell, _commodatum_) or has not yet done so (e.g., contract of sale in which title will pass later, on delivery or payment), the transferer has the risk and expense (see 1796).