(1)This section applies if an amount has been allowed as a deduction under section 579(2) by reference to the whole or part of—
(a)the periodical payment of interest, or other payment, mentioned in section 579(4)(a),
(b)the sum mentioned in section 579(6)(a), or
(c)the deemed payment of interest mentioned in section 579(7)(a).
(2)No further deduction is allowable by reference to all or part of the matched portion of the payment, sum or deemed payment.
(3)The “matched portion” of the payment, sum or deemed payment means—
(a)the whole of it, if the amount has been allowed as a deduction by reference to the whole of it, or
(b)the part of it by reference to which the amount has been allowed as a deduction, in any other case.