Is there a legally defined limit for gross overpricing (ghabn) that permits revocation?
Chapter on the Option of the Two Parties in Sale
Al-Mughni
Book of Sales
Primary text
There is no fixed definition for the threshold of ghabn mentioned in the established positions of Imam Ahmad. Abu Bakr in his 'Tanbih' and Ibn Abi Musa in 'Irshad' defined the threshold as one-third (al-thuluth). This is also the view of Malik, supported by the Prophet's saying: 'And one-third is a lot.'
Supporting text
It has been opined that the threshold is one-sixth (al-sudus). Another view holds that the threshold is whatever amount people customarily do not overcharge each other in, returning the determination to customary practice (urf) since the Lawgiver (Shari'ah) did not impose a specific limit.