Where -
(a) the period for which the credit or any part thereof is to be or may be provided cannot be ascertained at the date of the making of the agreement; and
(b) the rate or amount of any item included in the total charge for credit will change at a time provided in the transaction within one year beginning with the relevant date,
the rate or amount shall be taken to be the highest rate or amount at any time obtaining under the transaction in that year.