The general term “sale” extends to many kinds of transaction. Where legal evidence prohibits riba, a transaction containing riba is excluded from the general permission. The textbook presents this as a case of ʿamm whose scope has been specified.
General domain: transactions of sale.
Excluded case: a riba-bearing transaction.
Remaining rule: the basic permissibility of other sales meeting lawful conditions.
This does not imply that any profit or any difference in prices constitutes riba. Identifying riba is a separate legal inquiry.