What is the ruling regarding the introduction of a new defect (عيب) in the sold item?

Primary text

According to the narration of Al-Kharqi, which is one of the two established opinions from Ahmad, the buyer's word is accepted upon taking an oath regarding the emergence of a new defect. If the buyer takes the oath, recourse is against the trustee, and the trustee, in turn, seeks recourse against the mortgagor.