The treasury notes shall be sold by the state treasurer to the highest bidder for cash at not less than par and accrued interest at such times and in such amounts as may be determined by the commission, after advertising the time and place of sale in such manner as the commission shall determine; provided, the treasury notes or any part thereof may be sold by the state treasurer at any time at private sale without advertisement, for not less than par and accrued interest. The state treasurer, with the approval of the commissioner of public investments and other officials whose approval is required by law for investment of public funds, may purchase for investment any or all the treasury notes at par and accrued interest.