This paper asks how Gio. Domenico Peri's textbook Il Negotiante (1638-1665) reconciles Counter Reformation prohibitions on usury with the practical necessities of merchants for long distance trade, and what this reveal about continuities between Franciscan economic thought and contemporary business ethics discussion. Methodologically, it offers a narrative review and close reading of all four volumes of Il Negotiante, situating Peri within civil humanism, scholastic casuistry on usury, and the accounting tradition. The analysis shows how Peri domesticates credit and finance by reclassifying interest as compensation for identifiable services - risk bearing, delay, information, and intermediation - while condemning exploitative "usury." He translates theological tests into firm level bookkeeping, contract design, and institutional practices (exchange fairs, bills of exchange), making the ledger a site of moral discernment. The study argues that Peri's managerial casuistry bridges Franciscan concerns for reciprocity and charity with Pacioli inspired arithmetic pedagogy, offering a model of humanistic management where technical accounting and ethical judgment co-construct legitimate commerce. This perspective contributes to trace a Franciscan inflected lineage to early modern practices that negotiated faith, profit, and public trust.