Any building and loan association doing business in this State may borrow money from sources other than a Federal Home Loan Bank, and may secure the same by the assignment or pledge of any mortgage, mortgages, or other assets held by the building and loan association, but the amount borrowed from all such other sources shall not at any time exceed in the aggregate 30 percent of the shareholders' invested capital in said association. The amount borrowed from all sources shall at all times, irrespective of whether or not the same are secured, constitute a preferred claim superior to all claims on account of the shares of the building and loan association.