This paper stems from the conviction that Linguistics should not be limited to description and theorisation efforts, but also assume a more active and committed role in improving the exchanges that take place in the field of specialised or professional communication. The purpose of this article is twofold: to describe and characterize a prospective professional genre, the business agreement letter, and to provide specific knowledge about the linguistic expertise process on the basis of a relevant experience. From the Text Linguistics point of view (Schröder 1991; Adamzik 2004, 2016; Autora, 2003, 2016), and also taking into account the lexical-grammatical analysis, this article provides a generic description of the business agreement letter, with the aim of describing expectations and protecting the signatory parties from events that may take place in the future, which frequently result in interpretive problems. The case under analysis corresponds to an extensive legal and commercial controversy that arose in Argentina, as a result of a share purchase-sale contract, which required the collaboration of a linguistic expert. In this sense, it provides general knowledge about the linguistic expertise process and evidence on the relevance of linguistic advice services during the production and interpretation of professional texts.