Am I obligated to replace a part that I previously repaired at my own expense or at the customer's expense, if it breaks down after the agreed-upon warranty period has expired, and I am unable to determine the cause of the malfunction?

It is permissible to guarantee against a product defect for a specific period (performance guarantee) because it relies on testing and examining the product. Any defect that appears during this period is the seller's responsibility. The general principle in transactions is permissibility, and this guarantee reassures the buyer about the product's quality. There is a need for it, especially with the abundance of products and the difficulty of discovering defects at the time of purchase.

If a defect occurs after the experimental guarantee period, and the defect can only be attributed to manufacturing, then it is the seller's responsibility. If it can only be attributed to the buyer, then it is the buyer's responsibility.

If it could be attributed to either, then scholars have differed on whose word should be accepted. The majority hold that the seller's word, with an oath, is accepted, while the Hanbalis hold that the buyer's word, with an oath, is accepted.

To avoid taking an oath, the matter can be presented to the manufacturer or importer. Alternatively, the seller can sell the product with a condition of exemption from any defect after the trial period, or from the beginning of the contract. The correct view is that the condition of exemption is valid if the seller is unaware of the defect. However, if the seller knows about it, then he is a deceiver.