Whenever the board shall find that any disability beneficiary is engaged in or is able to engage in gainful occupation or work paying more than the difference between his disability retirement allowance and his average compensation, the board shall reduce his disability retirement allowance so that such allowance plus the amount earned by him, equals the amount of his average compensation. Should the earning capacity of any beneficiary be changed the board may further modify the amount of his disability retirement allowance so as to prevent such allowance plus the amount earned by the beneficiary from exceeding the amount of his average compensation.
T.C.A. § 8-3509