If an agreement can be negotiated for less than the appraised market value, purchase of such real estate is hereby authorized, without the exercise of the power of eminent domain, and the price agreed to may be made in one payment or in installments as hereinafter provided. Such lands shall be acquired in the name of the state of Kansas, and the state historical society shall not issue a voucher in payment therefor until the seller shall furnish an abstract showing a merchantable title, and a warranty deed conveying a fee simple title thereto to the state of Kansas nor until such abstract and deed have been approved by the attorney general: Provided, That in lieu of paying the full purchase price, the seller and the state historical society may enter into a lease-purchase agreement whereby payment for such property may be made in installments over a period not exceeding five (5) years. In the event such an agreement is entered into, it shall not become effective until approved by the attorney general, nor until such abstract and deed are approved as hereinbefore provided for nor until placed in escrow pursuant to an agreement approved by the attorney general.