“capital expenditure incurred”, for the purpose of determining the amount of capital expenditure incurred during any period in respect of any mine, means the amount (if any) by which the expenditure that is incurred during such period in respect of such mine and is capital expenditure, exceeds the sum of the amounts received or accrued during the said period from disposals of assets the cost of which has in whole or in part been included in capital expenditure taken into account (whether under this Act or any previous Income Tax Act) for the purposes of any deduction in respect of such mine under section 15(a) of this Act or the corresponding provisions of any previous Income Tax Act;