§ 931. Employees of the district attorneys in counties contained within the city of New York. The district attorney of each county contained within the city of New York is vested with the power to appoint any person to any position for which there is now provision by appropriation or which shall hereafter be established. All positions in the several district attorneys' offices within the city of New York for which there is now provision by appropriation shall be continued, except that the mayor of the city of New York may with the consent of the district attorney increase or decrease the number of positions and the term, grade, salary and compensation of any position.