What is the ruling on hiring transport for a fixed price per unit, with the remainder calculated proportionally based on that rate?

Primary text

It is permissible to hire transport for a fixed price for one unit, stating that whatever quantity exceeds that will be calculated according to the same rate. This is valid if the wording indicates the intent to transport the entire load, such as saying 'transport one qafiz for one dirham, and the rest according to that calculation,' or 'the remainder according to that calculation,' provided that this is understood from the wording or context.