In the computation of the gain consideration for the disposal shall be brought into account without any discount for postponement of the right to receive any part of it and, in the first instance, without regard to a risk of any part of the consideration being irrecoverable or to the right to receive any part of the consideration being contingent; and if any part of the consideration so brought into account is subsequently shown to the satisfaction of the inspector to be irrecoverable, such adjustment, whether by way of discharge or repayment of tax or otherwise, shall be made as is required in consequence.