Where entry tax is payable by a dealer or by any person in respect of the entry of goods into a local area comprised within the limits of a Cantonment Board and a tax in the nature of a tax or octroi is levied by the Cantonment Board on the entry of goods into that local area for consumption, use or sale therein, such dealer or person shall be entitled to a set off of an amount equal to either the tax proved to have been actually paid by him to the Cantonment Board or the tax payable under this Act, whichever is less:

Provided that no set off shall be admissible under this section unless such dealer or person furnishes the certified documentary evidence of having paid a tax or octroi recovered by the Cantonment Board.