(b)is transferred, whether in whole or in part, and the proceeds of the transfer (so far as they consist of capital sums) exceed the amount of the expenditure though incurred, but remaining unallowed, so much of the excess as does not exceed the difference between the expenditure incurred to obtain the licence or spectrum and the amount of such expenditure remaining unallowed, shall be chargeable to income-tax as profits and gains of the business in the tax year in which the licence or spectrum has been transferred;