Provided always, and be it further enacted, That the Releases and other Conveyances of Annuities or Rentcharges made in the original Grant thereof, subject to be redeemed or repurchased, shall, on the Repurchase thereof, be exempted from the ad valorem Duty hereby imposed on Conveyances on the Sale of any Property, and shall be charged only with the ordinary Duty on Deeds or Instruments of the like Kind not upon a Sale.