In case the cash value of such purchased business exceeds five hundred dollars, the directors of the association are authorized to hold the shares in excess of five hundred dollars in trust for the vendor or his assignee and dispose of the same to such persons, and within such times as may be mutually satisfactory to the parties in interest, and to pay the proceeds thereof as currently received to the former owner of said shares. Certificates of stock shall not be issued to any subscriber until fully paid, but the by-laws of the association may allow subscribers to vote as shareholders: Provided, part of the stock subscribed has been paid in cash.