§ 21

An officer of a city, town or district authorized to issue bonds or notes may provide that the bonds or notes of any issue may be called and redeemed, in whole or in part, prior to their maturity dates, at such price and under such terms and conditions as said officer may determine. No such bonds or notes shall be called and redeemed before their maturity dates unless funds, sufficient to pay the principal thereof, any redemption premium thereon and all interest accrued to the date fixed for redemption have been appropriated by the city, town or district and are then available; or, unless the city council of the city, the board of selectmen of the town or the prudential committee, if any, otherwise the commissioners of the district, shall have provided for issuance of refunding bonds or notes as provided in section twenty-one A.