Section 97. Annuities or increases of retirement allowances, pensions or annuities under sections ninety-five and ninety-six may be granted in any city by a two thirds vote of all the members of the city council and the approval of the mayor or city manager, as the case may be; and in a town by a two thirds vote of the voters at an annual town meeting upon recommendation of the selectmen. In a district increases under section ninety-six may be granted by a majority vote of the prudential committee, or board having similar powers. In a county increases under section ninety-six may be granted by a majority vote of the county commissioners.