The issue of Restitution as a duty of justice and as imperative of reason is recurrent and very differently treated by most of the Ibero-American Scholastic authors. In this article we address the peculiar perspective that the Dominican Tomás de Mercado gives to the subject in his Suma de Tratos y Contratos published in 1571. In this work, one of the first in the gender, Mercado emphasizing the obligatory aspect of Restitution as a moral virtue, and places doubt in favor of an eventual injured. He’ll defend the Restitution as an imperative of justice that binds in consciousness and which cannot be dispensed by any power since it is a mandate of the natural law.