With regard to usury-based transactions, barley and wheat are considered to be the same commodity. Therefore, if, for example, a person gives 1 kilogram of wheat and receives 1.5 kilograms of barley in return for it, it is usury and unlawful.
Also, if, for example, a person buys 10 kilograms of barley in return for 10 kilograms of wheat at the beginning of the harvest, then because he acquires the barley immediately but will give the wheat after some time, it is as if he has acquired something extra, rendering the transaction unlawful.