(1) On any debt or sum certain whereon interest is not reserved or agreed for, and which is overdue when the debtor is adjudged an insolvent, and which is provable under this Act, the creditor may prove for interest at a rate not exceeding six per centum per annum,—
(a) if the debt or sum is payable by virtue of a written instrument at a certain time, from the time when such debt or sum was payable to the date of such adjudication, or,
(b) If the debt or sum is payable otherwise, from the time when a demand in writing has been made giving the debtor notice that interest will be claimed from the date of the demand until the time of payment to the date of such adjudcation.
(2) Where a debt which has been proved under this Act includes interest or any pecuniary consideration in lieu of interest, the interest or consideration shall, for the purposes of dividend, be calculated at a rate not exceeding six per centum per annum, without prejudice to the right of a creditor to receive out of the debtor’s estate any higher rate of interest to which he may be entitled after all the debts proved have been paid in full.