A first mortgage upon real estate shall be given for every loan. If the loan is of a sinking fund type, it shall be accompanied by a transfer and pledge to the association of the shares borrowed upon, and all accumulations that have accrued or shall accrue thereon, as collateral security for the repayment of the loan. In lieu of the mortgage, the borrower or another may transfer and pledge to the association unpledged shares for the payment of the loan, the withdrawal value of which under the bylaws, at the time of such borrowing, shall exceed the amount borrowed and interest thereon for six months.