Where a portion of any land held under lease for the purposes of a deer forest or of a grouse moor, or for other sporting purpose, is assigned by the Crofters Commission for the enlargement of the holding or holdings of a crofter or crofters under this Act, the Crofters Commission shall, when they so assign such land, fix the amount of the deduction (if any) which in their judgment ought to be made from the rent payable by the tenant under the lease to the landlord, in respect of the portion of the land held under the same having been assigned as aforesaid, and thereafter the tenant under the lease shall be liable to the landlord only in the balance of the rent thereby stipulated, after deduction of the sum so fixed.