In the present research we propose the need for prior research on the phenomenological nature of money to carry out a clarifcation of the concepts that historical sciences (in this case, the history of economics) often project from the study of the sources. After showing the need for a distinction at the transcendental level between money as an entity that is «to buy» or as an entity that is «to pay», it will be shown how the establishment of this distinction could be fertile for methodological discussions. The example will be the scope and interpretation of the «Greek Law on Sale» by Fritz Pringsheim.