(7) (a) Unless otherwise agreed by the parties, where and in so far as an arbitral award is for the payment of money, the arbitral tribunal may include in the sum for which the award is made interest, at such rate as it deems reasonable, an the whole or any pan of the money, for the whole or any pad of the period between the date on which the cause of action arose and the date on which the award is made.
(b) A sum directed to be paid by an arbitral award shall, unless the award otherwise directs, carry interest at the rate of eighteen per centum per annum from the date of the award to the date of payment.