This work discusses the technique of forward tax substitution for a presumed taxable event, elaborating a description of the application of the Law, in which the individual guarantee of the ability to pay is related to the public interest that taxation be operationally viable. The methodological premises of the theory of principles are analysed, as well as the model for applying proportionality to the normative species and the structure and content of the right to ability to pay in the face of the Brazilian rules on forward tax substitution. This work also analyses the possibilities for giving greater fiscal justice to the legal-tax treatment of the issue.