Such bonds shall be sold in such manner and upon such terms as the governing body shall deem for the best interest of the borrower. In no event shall any of the bonds be sold on a basis to yield more than six per cent per annum from the date of sale to the date of average maturity of the bonds sold.
In any contract for the purchase or construction of any system or project named in SECTION 6-21-50 or for the improvement, enlargement, extension or repair of any such system or project provision may be made that payment therefor shall be made in such bonds.