The board shall have the right to lend to any employee member from the fund such an amount of money as will not cause the aggregate of indebtedness of the employee member to the fund immediately after such loan to exceed 50 percent of the amount of contribution returnable to him or her or to those claiming under him or her, were he or she to become separated from the service immediately after such loan. Interest on such loan shall be charged at the rate of six to seven percent per annum as determined by the board annually.