How is the oath administered when the buyer and seller dispute whether a defect in the sold item existed before or after the sale?
Chapter on Selling the Musarrah (Animal with milk retained in udder)
Al-Mughni
Book of Sales
Primary text
If the dispute over the defect's timing is of a type where only one party's claim is conceivable, such as an extra finger, a healed scar whose recurrence is impossible, or a fresh wound that cannot be old, then the claim of the claimant is accepted without an oath, as the truthfulness and falsehood of the parties are known, nullifying the need for an oath. If, however, both claims are plausible, such as a tear or patch in cloth, there are two narrations regarding the resolution. The first narration holds that the buyer's statement is accepted; he swears by Allah that he purchased it with the defect or that it did not occur while in his possession, and he retains the option for annulment or compensation. This is because the default is non-receipt of the deficient part and the right to the corresponding price, making the statement denying the claim prevail, similar to disputes over receipt of the sold item.
Supporting text
The second narration holds that the seller's statement prevails, provided he takes an oath. The oath must affirm the assertion definitively, not merely a lack of knowledge. If the seller claimed he sold it free of the defect, he swears to that. If he denies the buyer's right to annulment, he swears to that denial. This view is held by Abu Hanifa and Al-Shafi'i, grounded in the principle that the sold item is presumed sound and the contract valid, and the buyer asserts the right to annulment, which the seller denies; thus, the statement of the denier prevails.