Provided always, and be it enacted, That it shall not be lawful, either in any Court of Law or before any Justice of the Peace, to enforce the Payment of any Sum of Money claimed from any Driver or Conductor by any Proprietor on account of the Earnings of any Hackney Carriage or Metropolitan Stage Carriage, unless under an Agreement in Writing, which shall have been signed by such Driver or Conductor in the Presence of a competent Witness; and no such Agreement shall be liable to any Stamp Duty.