If the increase is deemed part of the price, its return is obligatory whenever the sale is reversed—whether because the object proves to be owned by someone else, because it is returned on account of a defect, or for any similar reason.
Likewise, whoever treats the increase as being governed by the ruling of the original price declares the sale corrupt whenever the increase itself is linked to a corrupt sale; whereas whoever does not treat the increase as part of the price imposes none of these consequences.
Abū Ḥanīfah held that the increase forms part of the price, yet he said that it is not taken into account with respect to the pre-emptive right (shufʿah) or in a murābaḥah sale; in those two cases the ruling reverts to the original price. Mālik adopted the same view.
Al-Shāfiʿī, however, maintained that neither an increase nor a reduction attaches to the price at all; in his view it is simply a gift.
Those who do attach the increase to the price cite the Almighty’s words:

“There is no blame on you if you mutually agree to anything after the prescribed obligation” (Q 4:24).

They argue: just as an additional sum agreed upon for the mahr becomes part of the mahr, so an increase agreed upon in a sale becomes part of the price.
The second group replies that consensus shows the increase is not taken into account in matters of pre-emption, and therefore it should not be linked to the price.
In short: whoever views the first contract as final says that the increase is a gift, whereas whoever regards it as a revocation of the first contract and the creation of a new one counts the increase as part of the price.

— SECTION FOUR — Disputes between Buyer and Seller

If the two parties agree that a sale has occurred but disagree over the amount of the price, and no evidence exists, the jurists of the various centres of learning unanimously hold that both parties must swear an oath and the contract, in principle, is annulled; yet they differ over the details—that is, the moment at which the oaths are taken and the annulment effected.

Abū Ḥanīfah and a number of scholars say: the parties swear oaths and annul the sale so long as the commodity itself has not perished; if it has perished, the buyer’s statement prevails, provided he swear an oath.
Al-Shāfiʿī, Muḥammad ibn al-Ḥasan (Abū Ḥanīfah’s disciple), and Ashhab (a disciple of Mālik) say: they may swear—and the sale is annulled—at any time.
As for Mālik, two reports are transmitted from him. One states that they swear and annul before possession; once possession has taken place the buyer’s word prevails. The second, related by Ibn al-Qāsim (with a parallel narration from Ashhab), aligns with Abū Ḥanīfah’s view; in this narration “loss” occurs through a change in market prices or through an increase or decrease in the value of the sold item.

Dāwūd and Abū Thawr, as well as Zafar, hold that the buyer’s statement prevails in every situation, except that Zafar makes an exception when the dispute concerns the very kind (jins) of the price—then, according to him, they must take oaths and rescind the sale.

There is no disagreement that, when the dispute concerns the very kind of the price or of the commodity, mutual oaths and rescission are obligatory. The jurists of the cities have nevertheless adopted, in general, the requirement of mutual oaths and rescission even when the dispute is only over the amount of the price, relying on the ḥadīth of Ibn Masʿūd in which the Messenger of Allah—peace and blessings be upon him—said:

“Whenever two people transact, the word is the seller’s, or they both return what they have taken.”

Whoever understands this ḥadīth as mandating rescission in all cases therefore says that, in every circumstance, the parties must swear oaths and annul the sale; his reasoning is that each of the two is both claimant and defendant at the same time.