Section 37A. Counties shall not issue any bonds or notes payable on demand, and they shall provide for the payment of all debts, except those incurred in anticipation of revenue or in anticipation of reimbursement from cities and towns, by such annual payments as will extinguish the same at maturity, and so that the first of such annual payments on account of any loan shall be made not later than one year after the date of the bond or note issued therefor, and so that the amount of such annual payment in any year on account of such debt, so far as issued, shall not be less than the amount of the principal payable in any subsequent year. The proceeds of any sale of bonds or notes, except premiums and accrued interest, shall be used only for the purposes specified in the authorization of the loan, and may also be used for costs of preparing, issuing and marketing such bonds or notes; provided, that the proceeds of any sale of bonds or notes for building, altering, furnishing or repairing public buildings or the construction or repair of public works may be used to pay management consultants hired under the provisions of chapter thirty B; and provided, further, that unexpended amounts may be applied to maturing annual payments of the same loan, and provided, further, that so much of such proceeds as has not been so applied at the expiration of two years from the completion of the project for which the loan was authorized shall become part of the next general unappropriated balance established under section twenty-nine or, if such loan was made on behalf of a district, shall be applied in reduction of assessments to be made upon it by the county. A treasurer of a county may invest not more than eighty per cent of the proceeds from the issue of bonds or notes prior to their application to the payment of liabilities incurred for the purposes specified in the authorization of the loan, in certificates of deposit in trust companies or national banks or in United States treasury bills. Any premium received upon such bonds or notes, less the cost of preparing, issuing and marketing them, shall be applied to the payment of the principal of the first bonds or notes to mature and any accrued interest received upon the delivery of said bonds or notes shall be applied to the payment of the first interest due thereon.