The creation of a tax system linking the interests of the crown as well as the elites constitutes one of the most important developments for the early modern State. It requires the setting up of a «fiscal constitution» in which all parties abide by certain rules, implicit or explicit, to ensure the correct functioning of the system. This article considers the profits arising from the surpluses that resulted from the collection of alcabalas by town councils on agreement of the Crown. This will be done by comparing the amount due to the crown in the encabezamiento with what was actually collected by the town of Madrid. So, it will be shown that the resulting surpluses were a constitutive part of this mutually beneficial system.