(c)which was or has been used for the purposes of business, is sold, discarded, demolished or destroyed and the moneys payable in respect of such building machinery, plant or furniture, as the case may be, together with the amount of scrap value, if any, exceeds the written down value, so much of the excess as does not exceed the difference between the actual cost and the written down value shall be chargeable to income-tax as income of the business of the previous year in which the moneys payable for the building, machinery, plant or furniture became due.