No stock-formed financial institution shall make a loan or discount on the security of the shares of its own capital stock, nor be the purchaser or holder of its shares, unless the security or purchase is necessary to prevent loss upon a debt previously contracted in good faith; provided, however, that whenever a financial institution makes a valid loan or discount in good faith upon the general credit of specific security or collateral of a shareholder, other than the shares in the financial institution, nothing in this section shall be construed to abridge or modify any of the provisions of the bylaws of the financial institution, as affecting the transaction, giving it a lien upon, or any other right or remedy relating to or affecting, the stock of the shareholder. All stock so purchased or acquired by the financial institution in good faith shall, within one year after its purchase, be disposed of or sold.