Ruling on exchanging wheat for wheat

Question:
The questioner Ramadan from Syria says: Is it permissible to exchange wheat for wheat by way of borrowing, knowing that the weight is known?

Answer:
If it is a loan that is repaid with the same as what was taken, if he borrows one hundred sa' or one hundred kilograms of wheat, he repays the same amount. And if he repays more without a condition, there is no harm: . As for taking a loan with an increase, that is not permissible; this is riba, such as taking one hundred sa' of wheat for one hundred and ten sa' of wheat, or one hundred sa' of rice for one hundred and ten sa' of rice; that is not permissible. But if he takes a loan, borrowing from his brother one hundred sa' of rice or wheat, and then repays the same amount, there is no blame. So praise be to Allah. And if he repays more without a condition, but merely as a voluntary act of kindness, there is no harm, due to his saying (peace and blessings be upon him): . And he (peace and blessings be upon him) would sometimes borrow and repay more.