Anti-Slavery Petition of 1790

In my study of our founding documents I found this Petition from the Pennsylvania Society for the Abolition of Slavery, signed by Benjamin Franklin and published in 1790, to be rather interesting. In the Letter the society asks Congress:

that you will be pleased to countenance the Restoration of liberty to {slaves}, that you will devise means for removing this Inconsistency from the Character of the American People, that you will promote mercy and Justice towards this distressed Race, & that you will Step to the very verge of the Powers vested in you for discouraging every Species of Traffick in the Persons of our fellow men. (emphasis added, capitalization original)

I wondered what they expected Congress to do considering the prohibitions on Congress’ power regarding slavery prior to 1808 as stated at the beginning of Article 1 Section 9 of the Constitution:

The migration or importation of such persons as any of the states now existing shall think proper to admit, shall not be prohibited by the Congress prior to the year one thousand eight hundred and eight, but a tax or duty may be imposed on such importation, not exceeding ten dollars for each person.

I assume that stepping to the very verge of the powers vested in Congress would mean that they wanted Congress to impose a tax of $10 per slave on the importation of new slaves.