Sec. 6. ASSIGNMENTS OF SALARY TO FUND. Any members who have or may have any back or past due salary due them, from any such city, may assign all or any portion of such back salary to said pension fund, and such assignments as have or may hereafter be executed by any such members, are hereby validated and shall be recognized by the governing body of any such city, and such sums, if any, shall be paid into the said pension fund.