Is the sale of an object containing a substance subject to Riba (usury) permissible if exchanged for a different genus of that substance, where the Riba substance is incidental?

Primary text

The transaction is valid if an object subject to Riba is exchanged for a different genus thereof, provided the Riba-susceptible element is not the primary object of the sale, such as a house whose roof is overlaid with gold. This is established without known dispute. Similarly, exchanging a house whose roof is overlaid with gold or silver for another such house is permissible because the Riba-susceptible material is incidental, rendering its presence equivalent to its absence in the transaction's validity. This principle extends to purchasing a slave who possesses wealth, where the condition of transferring that wealth (which is of the same genus as the price) is permissible if the wealth is not the intended object of the sale. Likewise, exchanging a slave for a slave, with the condition of transferring each slave's wealth, is permissible if the wealth is not the primary intent, resembling the incidental overlay on the roof. Consequently, the sighting of the wealth is not required for the sale's validity or binding nature.