Several instruments used in single transaction of sale, mortgage or settlement. — (1) Where, in the case of any sale, mortgage or settlement, several instruments are employed for completing the transaction, the principal instrument only shall be chargeable with the duty prescribed in Schedule I, for the conveyance, mortgage or settlement, and each of the other instruments shall be chargeable with a duty of one rupee instead of the duty (if any) prescribed for it in that Schedule.