Where a sum is recoverable from an authority under section 257 or 258 of this Act by reference to an acquisition or purchase of an interest in land, and in respect thereof, or of a subsequent appropriation of the land, a grant became or becomes payable to that or some other authority under an enactment, the power conferred by that enactment to pay the grant shall include, and shall be deemed always to have included, power to pay a grant in respect of that sum as if it had been expenditure incurred by the acquiring authority in connection with the acquisition or purchase.