No agreement which tends to diminish or defeat the right, title or interest of the guaranty credit union in any asset acquired by it, either as security for a loan or by purchase, shall be valid against the guaranty credit union unless such agreement shall be in writing; shall have been executed by the credit union and the person or persons claiming an adverse interest thereunder, including the obligor, contemporaneously with the acquisition of the asset by the credit union; shall have been approved by the board of directors of the credit union with such approval reflected in the minutes of said board; and shall have been, continuously, from the time of its execution, an official record of the credit union.