If one of the conditions of a valid transaction (muʿāmalah) is not fulfilled in a sale – for example, an item that must be bought and sold by weight is sold without weighing it – the transaction is invalid (bāṭil). Despite this, in the event that the seller and the buyer consent to the other having disposal over the property, there is no problem. Otherwise [i.e. if they do not consent], the thing they have taken from each other is like usurped property and must be returned to the other.
In case the property of one of them perishes while it is in the possession of the other, the latter must replace it, whether he knows the transaction was invalid or not.