Both the revenue bonds and general obligation serial bonds may be sold at not less than par at public sales held after notice published prior to such sale in a newspaper having a general circulation in the county and in such other medium of publication as the council may determine, or may be exchanged for other bonds on the basis of par; provided, that such bonds may be sold to the federal government at private sale at not less than par, and, in the event less than all of the authorized principal amount of such bonds is sold to the federal government, the balance may be sold at private sale at not less than par at an interest cost to the county not to exceed the interest of the portion of the bonds sold to the federal government. (Mont. Co. Code 1965, § 26-11; 1961, ch. 821, § 1.)