The fundamental concept of substance traverses all phases of philosophy and science. And in conformity with the loftiness and maturity of thought that Leibniz achieved with his principle of living force, Kant was able to break away from all scholasticism with regard to the concept of substance, and to make it a presupposition for the concepts of relation. The position that Kant gave to substance as a precondition for causality and reciprocity of action tore away, as it were, the absolute independence of substance. It has absoluteness as a category only as a "precondition" of causality. This absoluteness does not rest in the category itself, is not confined to it itself, but realizes itself only insofar as it makes causality possible, for the latter could not do its work without the former.
Hermann Cohen, Religion of Reason out of the Sources of Judaism, p. 60. As stated I think this is somewhat misleading; the idea that substance is a precondition of causality and reciprocity of action is itself scholastic, by way of Wolff and Leibniz, and the idea of substance as having absolute independence is Cartesian (although Descartes was himself adapting scholastic ideas). It's not the content here so much as the way in which Kant proceeds in order to establish it that is the real divergence.