All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
It is permissible for the person who bought a commodity without knowing that it is defective to return it to the seller in case he found about that defect after the sale contract was concluded and the defect itself didn`t happen after he took possession of that commodity. It is also incumbent upon the seller to terminate the sale contract, whether there was a defect in the commodity itself or it didn`t have the features he had mentioned to the buyer.
The evidence on this is that Aisha, Ummul Mu'minin, narrated:
A man bought a slave, and he remained with him as long as Allah wished him to remain. He then found defect in him. He brought his dispute with him to the Prophet (PBUH) and he returned him to him. The man said: Messenger of Allah, my slave earned some wages. The Messenger of Allah (PBUH) then said: Profit follows responsibility. {Musnad Ahmad}.
Moreover, it is stated: "Generally speaking, there is no disagreement among the jurists about the permissibility of returning the defective commodity to the seller." [Al-Mawso`ah al-Fiqhiyah, vol.20/pp.113]
In conclusion, We (Iftaa` Department) advise the parties mentioned in the above question to seek the judgment of a knowledgeable scholar to ward off dispute and conflict. And Allah The Almighty Knows Best.