A promissory note is a financial instrument which, as is defined in the question, consists on a written promise made by one party (the issuer), to pay a fixed amount on a certain due date to another party (the payee or receiver). Any terms concerning the repayment are agreed between the parties and contained in the document.
Moreover, it is a negotiable instrument and the debt can be transfered. Therefore, Onyx may transfer the right to receive money to a third party as part of its business activities. The promise stills there but the parties are not the same, therefore the note has been negotiated.
