Catholic Church’s usury restrictions and their effects on business life have long attracted scholars’ attention. Several essays reconstruct the evolution of usury doctrine and its relations with the diffusion of new contracts and financial techniques to enable the payment of interest. This paper aims to investigate the real influence of the Church’s precepts on financial transactions starting from a different observation point. The most important sources used in this work are commercial arithmetic textbooks printed in Italy from the beginning of the 16th century to the end of the 18th century and specifically devoted to businessmen. Although quite limited from a theoretical point of view, these books reflect coeval business practices. In particular, they contain several examples illustrating calculation techniques, including problems concerning interest and discount. Many authors try to give suggestions to their readers, as well as some ethical and moral advice, showing whether, in their opinion, a specific financial practice was licit or not. Therefore, these textbooks represent a useful source to shed light upon the influence of the Church’s usury precepts on the business world in the early modern age.