Buying and Selling: Selling Different Commodities Sold by Weight or Measure
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2085
With regard to things that are sold by weight or measure, if the thing that is sold and the payment in exchange for it are not of the same commodity and the transaction does not have a period, there is no problem in taking more. However, if the transaction has a period, it is problematic [i.e. based on obligatory precaution, it is not valid]. Therefore, if 1 kilogram of rice is sold for 2 kilograms of wheat after one month, the validity of the transaction is problematic [i.e. based on obligatory precaution, it is not valid].
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling