What is the evidentiary weight of the seller's oath regarding a claimed error in the original cost?
Chapter on Selling the Musarrah (Animal with milk retained in udder)
Al-Mughni
Book of Sales
Primary text
The oath mentioned by Al-Khiraqi in this context is one taken by the seller to affirm that at the time of the sale, they were unaware that the purchase price was higher than stated. If the seller makes this oath, the buyer is given the option: either to accept the item at the agreed upon price plus the claimed error amount (with the excess being deducted from the profit margin), or to annul the contract. This option is granted because the buyer entered the contract under the assumption of a specific total price; if the actual price is higher, it results in a detriment to the buyer, akin to purchasing a defective item.
Supporting text
If the seller claims the buyer knew of the original error and the buyer denies it, the buyer's word is accepted. The correct opinion is that the buyer must swear an oath affirming their lack of knowledge of the original higher price, as the seller is claiming something that would obligate the buyer to either return the item or pay an increased price. If the buyer refuses the oath (nakul), judgment is passed against them.