An authority may sell or exchange the capacity or output of a project not then required by any of its member governmental units for such consideration, for such period, and upon such other terms and conditions as may be determined by the parties, to any person, firm, association or corporation, public or private within or outside the Commonwealth; however, this shall not authorize retail sales by an authority to any nongovernmental end user of electric capacity or energy, except as set forth in § 15.2-5406.1, and sales of such capacity or output of a project shall not be made in such amounts, for such periods of time, and under such terms and conditions as will cause the interest on bonds issued to finance the cost of a project to become taxable by the federal government.