Moreover, the foundational basis of contracts is the mutual consent of the contracting parties, and their binding force consists in what the parties have made binding upon themselves by entering into the contract. Allah says in His Noble Book: “Unless it is trade conducted by mutual consent among you” [al-Nisā’: 29] [al-Nisā’: 29]. He also says: “But if they willingly remit any of it to you, then consume it with pleasure and ease” [al-Nisā’: 4]. He made the permissibility of consuming it conditional upon their willing consent, just as a consequence is made conditional upon its condition. This demonstrates that such consent is its effective cause. Here, the ruling is attached to a suitable, derived attribute, which demonstrates that this attribute is the effective cause of that ruling. Since willing consent makes it permissible to consume part of the bridal gift, the same applies to all other voluntary gifts, by analogy with it on the basis of the expressly indicated effective cause established by the Quran. Likewise, in His statement, “Unless it is trade conducted by mutual consent among you” [al-Nisā’: 29], He stipulated nothing for trade other than mutual consent. This entails that mutual consent is what makes trade permissible. Accordingly, whenever two contracting parties mutually consent to a transaction, or a donor willingly makes a gift, its permissibility is established by the Quran—unless it involves something Allah and His Messenger have prohibited, such as trading in wine or the like.

Moreover, a contract may exist in either of two states: unqualified or qualified. One must therefore distinguish between an unqualified contract and the essential meaning common to contracts as such. Thus, if it is said, “This condition conflicts with what the contract entails,” and what is meant is that it conflicts with an unqualified contract, then the same is true of every additional condition, and this does not impair the contract. If, however, what is meant is that it conflicts with what the contract necessarily entails in both its unqualified and qualified forms, evidence is required to establish that. Such a claim is valid only when the condition conflicts with the purpose of the contract. If a contract has a purpose sought in every form it takes, and a condition contrary to that purpose is stipulated within it, two contradictory things have been combined: affirming the purpose and negating it. Nothing can therefore be achieved. Scholars unanimously agree that such a condition is void; indeed, in our view, it also invalidates the contract itself.

A condition may also be invalid because it conflicts with the Lawgiver’s purpose. An example is stipulating that the right of patronage belongs to someone other than the person who emancipates the slave, for this conflicts neither with what the contract entails nor