No, accepting partial payment (e.g. the previous normal amount without the balloon portion) does not constitute waiver of your right to the full (e.g. with balloon) payment; a creditor is allowed to accept any amount less than the full amount due while seeking the rest of the money. It would probably be a good idea to send your debtor a reminder about the balloon payment; if they still don't pay, you can try to work it out with them (e.g., maybe they'll pay if given a payment plan or schedule) or could file a lawsuit against them for breach of contract (since a promissory note is also a contract).