Any member bank may contract for, or purchase from, any of its directors or from any firm of which any of its directors is a member, any securities or other property, when (and not otherwise) such purchase is made in the regular course of business upon terms not less favorable to the bank than those offered to others, or when such purchase is authorized by a majority of the board of directors not interested in the sale of such securities or property, such authority to be evidenced by the affirmative vote or written assent of such directors: Provided, however, That when any director, or firm of which any director is a member, acting for or on behalf of others, sells securities or other property to a member bank, the Board of Governors of the Federal Reserve System by regulation may, in any or all cases, require a full disclosure to be made, on forms to be prescribed by it, of all commissions or other considerations received, and whenever such director or firm, acting in his or its own behalf, sells securities or other property to the bank the Board of Governors of the Federal Reserve System, by regulation, may require a full disclosure of all profit realized from such sale. Any member bank may sell securities or other property to any of its directors, or to a firm of which any of its directors is a member, in the regular course of business on terms not more favorable to such director or firm than those offered to others, or when such sale is authorized by a majority of the board of directors of a member bank to be evidenced by their affirmative vote or written assent: Provided, however, That nothing in this section contained shall be construed as authorizing member banks to purchase or sell securities or other property which such banks are not otherwise authorized by law to purchase or sell.