Section 96. A city, town, district, or county may increase to an amount not exceeding twelve hundred dollars the annual amount of any retirement allowance, pension, annuity or other benefit, payable by it under any general or special law, to any former official or employee, including an employee of a housing authority, or to his dependent as provided in such general or special law, which is less than twelve hundred dollars; provided, that such official or employee had been in the service of such city, town, housing authority, district, or county for not less than fifteen years.