Where—
a sum is recoverable from any authority under section 308 . . . by reference to an acquisition or purchase of an interest in land, and
a grant became or becomes payable to that or some other authority under an enactment in respect of that acquisition or purchase or of a subsequent appropriation of the land,
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.