Is it permissible to exchange walnuts in their shells for shelled walnuts of an agreed-upon weight and quantity?

Selling walnuts (with their shells) for shelled walnuts is a matter of disagreement among the different schools of thought:

Malikis and Hanbalis: They believe that walnuts are not subject to Riba (usury), and therefore, it is permissible to sell them for their shelled kernels, whether in equal or unequal amounts. Shafi'is: They do not permit selling walnuts for the kernels derived from them. Hanafis: They permit selling walnuts for their shelled kernels on the basis of "consideration," meaning that the pure shelled kernels given in exchange must be greater in amount than the kernels contained within the walnuts being given.

To avoid disagreement, it is preferable to refrain from such a sale. However, there is no harm for one who performs it, following the opinion of those who permit it.