The comptroller or the disbursing officer of an agency that has the power to invest assets directly may pay for authorized securities purchased from or through a member in good standing of the National Association of Securities Dealers or from or through a national or state bank on receiving an invoice from the seller of the securities showing that the securities have been purchased by the board or agency and that the amount to be paid for the securities is just, due, and unpaid. A purchase of securities may not be made at a price that exceeds the existing market value of the securities.