Section 20. The proceeds of any sale of bonds or notes shall be used only for the purposes specified in the authorization of the loan; provided, however, that such proceeds may also be used for costs of preparing, issuing and marketing the bonds or notes, except as otherwise authorized by this section. If a balance remains after the completion of the project for which the loan was authorized, the balance may at any time be appropriated by a city, town or district for any purposes for which a loan may be incurred for an equal or longer period of time than that for which the original loan, including temporary debt, was issued. Any balance not in excess of $50,000 may be applied, with the approval of the chief executive officer, for the payment of indebtedness. If a loan has been issued for a specified purpose but the project for which the loan was authorized has not been completed and no liability remains outstanding and unpaid on account thereof, a city, by a two-thirds vote of all of the members of the city council, or a town or district, by a two-thirds vote of the voters present and voting thereon at an annual town or district meeting, may vote to abandon or discontinue the project and the unexpended proceeds of the loan may be appropriated for any purpose for which a loan may be authorized for an equal or longer period of time than that for which the original loan, including temporary debt, was issued. Any premium received upon the sale of the bonds or notes, less the cost of preparing, issuing and marketing them, and any accrued interest received upon the delivery of the bonds or notes shall be: (i) applied, if so provided in the loan authorization, to the costs of the project being financed by the bonds or notes and to reduce the amount authorized to be borrowed for the project by like amount; or (ii) appropriated for a project for which the city, town or district has authorized a borrowing, or may authorize a borrowing, for an equal or longer period of time than the original loan, including any temporary debt, was issued, thereby reducing the amount of any bonds or notes authorized to be issued for the project by like amount. Notwithstanding this section, no appropriation from a loan or balance thereof shall be made that would increase the amount available from borrowed money for any purpose to an amount in excess of any limit imposed by general law or special act for that purpose. Additions to the levy limit for a debt exclusion are restricted to the true interest cost incurred to finance the excluded project.