(ii)nothing in Clause (b) shall apply to a contract by which a raiyat binds himself to pay an enhanced rent in consideration of an improvement, which has been or is to be effected in respect of the holding by, or at the expense of, his landlord, and to the benefit or which the raiyat is not otherwise entitled; but an enhanced rent fixed by such a contract shall be payable only when the improvement has been effected and, except when the raiyat is chargeable with default in respect of the improvement, only so long as the improvement exists and substantially produces its estimated effect in respect of the holding;
