When the rent of any tenant has been enhanced under section 13 or was fixed at the current settlement with regard to an improvement made by or at the expense of the landlord, a Revenue-officer may at any time, on the application of the tenant, modify or cancel the order for enhancement, or reduce such rent, on the ground that the effect of the improvement in increasing the productive power of the holding has diminished or ceased since the date of the order for enhancement or of the last modification of such order made under this section, or since the rent was fixed by the Settlement-officer, as the case may be.