Except by procedures provided in this Charter, the terms of the elective officers of the City shall not be shortened during the term for which he was elected. The terms of elective officers of the City may not be extended beyond the period for which any such officer was elected, except that an elective officer of the City may, after his term has expired, continue to hold office until his successor is elected or appointed and has qualified. The Council shall not grant or authorize extra compensation to any City officer, agent or contractor after the service has been rendered or the contract entered into; nor shall the salary of any elective City officer be increased or decreased after his election during any fixed term of office for which he was elected.