The proceeds of bonds issued pursuant to § 15.2-5125 shall be used solely for the payment of the cost of the system or systems for which they were issued and shall be disbursed in such manner and under such restrictions, if any, as the authority may provide in the authorizing resolution or in any trust agreement. If the proceeds of the bonds, by error of estimates or otherwise, are less than such cost, additional bonds may in like manner be issued to provide the amount of such deficit and, unless otherwise provided in the authorizing resolution or in the trust agreement securing them, shall be deemed to be of the same issue and entitled to payment from the same fund without preference or priority of the bonds first issued for the same purpose. If the proceeds of the bonds of any issue exceed the amount required for the purpose for which such bonds were issued, the surplus shall be deposited to the credit of the sinking fund for such bonds.