“I promise to pay B ₹500 and all other sums which shall be due to him.” Under the Negotiable…

A promissory note must undertake payment of a certain sum of money only. The additional promise to pay all other sums that may become due makes the total uncertain; section 4 illustration (d) gives this exact example as not a promissory note. It is not an order addressed to a drawee, so it is not a bill of exchange. Section 17’s ambiguous-instrument rule applies when an instrument can be construed as either a note or a bill, not where the essential certainty requirement is absent.