Said debentures 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 New Mexico compilation commission, after advertising the time and place of sale in such manner as the commission may determine. Provided, however, that said debentures 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 may, with the approval of the state board of finance, and other officials whose approval is required by law for investment of public funds, purchase any or all of such debentures at not less than par and accrued interest without the necessity of advertising or offering said debentures for public sale or after rejection of bids for all or part of any issue.