Any building and loan association is hereby authorized, with the approval of its board of directors, to assign its mortgages and the evidence of debt secured thereby to the Home Owners' Loan Corporation created by act of Congress of the United States under the act cited as the Home Owners' Loan Act of 1933, or such other corporation as may be created by authority of the United States Government, or as an instrumentality of the United States Government, and to accept as consideration for such assignment, cash or bonds of such Home Owners' Loan Corporation or such other corporation as may be created by authority of the United States Government, or as an instrumentality of the United States Government; Provided, that no mortgage given by any member of such association shall be so assigned without the written consent of the borrowing member.