If the sum, which in the opinion of the Authority concerned, the Collector ought to have awarded as compensation is in excess of the sum which the Collector did award as compensation, the award of the Authority concerned may direct that the Collector shall pay interest on such excess at the rate of nine per cent. per annum from the date on which he took possession of the land to the date of payment of such excess into Authority:
Provided that the award of the Authority concerned may also direct that where such excess or any part thereof is paid to the Authority after the date or expiry of a period of one year from the date on which possession is taken, interest at the rate of fifteen per cent. per annum shall be payable from the date of expiry of the said period of one year on the amount of such excess or part thereof which has not been paid into Authority before the date of such expiry.