The proceeds and investments in escrow or trust shall be in an amount at the time of issuance of such refunding bonds which is certified by a certified public accountant licensed to practice in this state to be sufficient, together with any interest or other gain to be derived from any such investment, to pay the principal of and interest and redemption premiums, if any, on the refunded bonds as they become due or at designated dates prior to maturity (in connection with which the legislative body has exercised or has obligated itself to exercise a redemption privilege on behalf of the city), and the designated costs of issuance of the refunding bonds.