Madison was referring to the future imposition of duties on imports and exports.
"Were these at liberty to regulate the trade between State and State, it must be foreseen that ways would be found out to load the articles of import and export, during the passage through their jurisdiction, with duties which would fall on the makers of the latter and the consumers of the former. We may be assured by past experience, that such a practice would be introduced by future contrivances; and both by that and a common knowledge of human affairs, that it would nourish unceasing animosities, and not improbably terminate in serious interruptions of the public tranquillity."
-- Federalist #42
What are the specifics of the case? What case was this that you think it evidence of the Founder's intent to grant the power to RCATSS for the "positive purposes of the General Government"?