BSD Ribbis in context of business. Case: A merchant receives a prepayment from a client who lives in an expensive city, in order to bring him merchandise from a cheaper place. E.g. Client gives merchant $100 in a city where $1000 can buy him 100 kg of wheat, in order that the merchant will travel to a place where $1000 buys him 130 kg of wheat. Is there interest hidden in this deal? And if yes, how can it be avoided?