An association may acquire all or at least 90 percent of the issued and outstanding stock of another association, or any other corporation, with the prior written approval of the commissioner upon conditions that the commissioner may impose as a step in a plan of merger to be approved by the commissioner. The acquisition of stock shall be upon terms as may be approved by the board of directors of the acquiring association and ratified by the vote or written consent of holders of a majority of the outstanding stock of each class of stockholders of the acquiring association.