All perfect praise be to Allah, The Lord of The Worlds; and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
If the sale contract was concluded fulfilling its Sharia conditions and pillars, then it is binding. However, if the agreed upon conditions haven`t been fulfilled after the sale contract was concluded, both parties may withdraw from the contract. Muslim jurists have confirmed actio empti in favor of the buyer. In other words, both parties may withdraw from the sale contract if a defect was detected in the price or the commodity, and which could decrease the value of any since the conditions agreed-upon in the contract haven`t been fulfilled.
Therefore, if a defect was discovered by any party after the conclusion of the contract, and that defect wasn`t old, then he may return the commodity to its original owner who becomes obliged to invalidate the sale contract, whether the sold item was defective or haven`t met the specifications stated by the seller. This is based on the Hadith of the Prophet(PBUH) who said, " 'The Muslim is the brother of another Muslim, and it is not permissible for a Muslim to sell his brother goods in which there is a defect, without pointing that out to him."'{Ibn Majah}. Al-Imam An-Nawawi said, "It is permissible for the buyer to return the commodity if he discovered that it had an old defect." {Minhajj At-talibeen(1/100).
The above ruling applies whether there is a warranty or not i.e. the emergence of an old defect obligates the seller to account for it by withdrawing from the contract or replacing the commodity, whether he had guaranteed it or not, unless he stipulated that he is free from the liability of all the defects, in case they emerged, and the buyer had approved of that. Only then, the seller is free from the liability and the buyer may not withdraw from the contract because he had agreed on the condition of the seller.
Ibn Al-Hammam said, "The sale transaction concluded with the condition of being absolved of defects is valid. The seller is free from the liability for every defect present at the time of the sale, whether he knew about it or didn`t."{Fatih Al-Khadeer Li Al-Kamal Ibn Hammam(6/397).
Moreover, this has been stipulated in article(194) of the Jordanian Civil Law. As a result, if the seller hadn`t conditioned that he is free from the liability with regard to the defects of the commodity, then he is obliged to take that dynamo back and refund the buyer. And Allah knows best.