Section 26. No corporation subject to this chapter shall pay any salary, compensation or emolument to any officer, trustee or director thereof, nor any salary, compensation or emolument amounting in any year to more than twenty thousand dollars to any person, unless such payment be first authorized by a vote of its board of directors; provided, however, that the amount of such salaries, compensation or emoluments may be adjusted by the corporation to exceed such amount to reflect any decrease or increase in the cost of living since January first, nineteen hundred and sixty-nine, as measured by the Consumer Price Index for Urban Wage Earners and Clerical Workers, United States City Average, published by the Bureau of Labor Statistics of the United States Department of Labor. No corporation subject to this chapter shall make any agreement with any of its officers, trustees or employees whereby it agrees that for any services rendered or to be rendered he shall receive any salary compensation or emolument for a period of more than three years from the date of such agreement; provided, however, that the payment of an amount not in excess of twelve and one half per cent of such annual salary, compensation or emolument, or a larger percentage if approved by the commissioner, may be deferred beyond such period.