Payment by promissory note is only permitted to the debtor if it has been expressly provided for by the parties and mentioned on the invoice. Even in this case, if the promissory note has not reached the creditor within thirty days of the invoice being sent, the creditor may issue a bill of exchange which the debtor is obliged to accept under the conditions set out in the penultimate and final paragraphs of Article L. 511-15. Any stipulation to the contrary is deemed unwritten.