In order to fulfill its purpose, the Administration is obliged to resort to the so-called public-private alliance, due to which, the first commissions the second, which, adjusting both to the administrative contracting regulations and to the commercial one, collects through the use of an invoice. In this context, a recent sentence issued by the Supreme Court is critically commented, which, advocating the existence of a normative contradiction, confirms the first-degree ruling that rejects the exceptions opposed by the executed Municipality, ruling on its qualification, and through said intermediary, generating a normative precedence concerning the legal orders that regulate the matter. The gloss will try to provide a correct interpretation of the regulatory scheme of the invoice in the framework of the administrative activity, as well as to correctly qualify the exception of the contract not fulfilled.