If the board of trustees finds that any disability beneficiary, receiving benefits under the provisions of N.H. Rev. Stat. Chapter 100 or RSA 192, is engaged in or is able to engage in a gainful occupation paying more than the difference between his retirement allowance and the annual amount of compensation currently payable for the job classification last held by the beneficiary prior to his disability retirement, then his state annuity shall be reduced to an amount which, together with his member annuity and the annual amount earnable by him, is equal to the annual amount of compensation currently payable for the job classification last held by the disabled beneficiary. Such reduction shall not be made if the disabled beneficiary demonstrates to the satisfaction of the board of trustees that the annual compensation currently earnable by him does not exceed the difference between his retirement allowance and the annual amount of compensation currently payable for the job classification which he last held prior to his disability retirement. If his earning capacity is later changed, his state annuity may be further modified; provided that the new state annuity shall not exceed the amount of the state annuity originally granted nor an amount which, when added to the amount earnable by him, together with his member annuity, equals the annual amount of compensation currently payable for the job classification last held prior to his disability retirement. It shall be the responsibility of the disability beneficiary to file with the board of trustees, annually on a date determined by the board, proof of the annual amount of compensation currently earnable by him and of the annual amount of compensation currently payable for the job classification last held by him prior to his disability retirement.