What is the ruling on selling a bag of onions on condition of returning it as two bags?

Onions are a ribawi (usurious) commodity according to the Shafi'i and Maliki schools because they are used to prepare food or storable provisions, and they are assimilated to salt, which is explicitly mentioned in the Hadith. Therefore, it is impermissible to sell onions for onions except under two conditions: equality in quantity and immediate mutual possession at the time of the contract. As for onion seeds, they are not a ribawi commodity, and it is permissible to sell and plant them for their own kind, even with a difference in quantity or on credit.