The resolution shall direct the county treasurer to sell the refunding bonds on a date not less than five days, nor more than 10 days, prior to the April 1st or October 1st prior to the maturity of the bonds to be refunded, and shall declare whether the refunding bonds may be sold for less than the face value thereof, plus accrued interest. In no event shall refunding bonds be sold for less than 90 percent of their face value plus accrued interest.