The governing authority of the parish may sell the bonds in the manner and for the price which it determines to be for the best interests of the parish, but no such sale shall be made at less than par.  The proceeds of the sale of the bonds shall be used solely for the payment of the cost of the bridge or bridges and shall be disbursed under such restrictions, if any, as the governing authority provides.  

If the proceeds of the bonds, by error or calculation or otherwise, are less than the cost of the bridge or bridges, additional bonds may in like manner be issued to provide the amount of the deficit.  Unless otherwise provided in the mortgage or the trust indenture hereinafter mentioned, these latter bonds shall be considered to be of the same issue and shall be entitled to payment from the same fund without preference or priority of the bonds issued first.  If the proceeds of bonds issued for any bridge or bridges exceed the cost thereof, the surplus shall be paid into the sinking fund provided for the payment of principal and interest of the bonds.