He also said—may Allah have mercy on him:

Section: Property Received under an Invalid Contract

The underlying principle is that a valid contract obliges each contracting party to fulfil its requirements. Thus, sales, leases, marriage contracts, and similar binding contracts of exchange require each party to deliver what is due to the other. Their binding nature entails the obligation to fulfil them and the prohibition against breaking them.

As for “revocable contracts,” including all forms of agency and partnership, they do not entail an unconditional obligation of fulfilment. Such a contract is not binding such that it must be fulfilled; rather, it is revocable and permissible, and the party concerned may choose either to maintain it or to terminate it. Terminating it brings it to an end. Yet, as long as the contract remains in force, he must fulfil its requirement to safeguard the property, for it is a contract of trust.

As for the prohibition of wrongdoing, such as betrayal of trust, that obligation is imposed by the revealed law, not by reason: he is forbidden to wrongfully encroach upon the property of anyone, whether that person has entrusted him with it or not. Nevertheless, the contract also imposes this obligation and further reinforces it.