The second fundamental principle is this:

If the two parties consent only on the basis of a stipulation that conflicts with Allah’s Book, they have undertaken something Allah has forbidden. Their undertaking is therefore not binding, just as a vow to commit a sin is not binding. This applies whether they know of the prohibition or are unaware of it. If one party imposes such a stipulation on the other, believing it permissible, and consents only on that basis, the contract is not binding upon him—unless he undertook it for Allah’s sake. In that case, whatever was undertaken for Allah’s sake remains binding, but not what was not. This applies to vows, endowments, bequests, and other arrangements whose components can be treated separately.

If, however, he knows that the stipulation is forbidden and nevertheless imposes it, it is like the stipulation made by Barīrah’s owners: his stipulation is invalid, but the contract is not invalidated. In this respect, there is no distinction between marriage, sale, and other contracts.

Some jurists have invalidated many stipulations in marriage without evidence. Then, regarding an invalid stipulation in marriage, they say that the stipulation is invalid but the marriage remains valid without it—even though the party imposing the stipulation consented to the marriage only on that basis. Yet stipulations in marriage are more binding than those in a sale, because the Prophet (peace and blessings be upon him) said:

“The stipulations most deserving of fulfilment are those by which you have made sexual relations lawful.”

Their position has thus led them to contradict the scriptural texts in many instances and to bind people to something they neither undertook nor were bound to by Allah. They have therefore imposed upon people what Allah and His Messenger did not impose. They may then take a broad approach to divorce, which Allah detests, thereby forbidding people what Allah and His Messenger have not forbidden. Then they permit it through contracts containing invalid stipulations, thereby making lawful what Allah and His Messenger have not made lawful.

For example, stipulating in the marriage contract that the woman is to be made lawful again for her former husband is, by consensus, a forbidden and invalid stipulation.