I would say that this note would still be valid despite no date on it since it states that it will be paid after the sale of his automobile and in the absence of a date, I believe it could be considered be paid up to 6 months after the sale of the automobile since that sale will have a date or needs to be ensured that such a date is now recorded. While a full signature is preferable, I believe the initial will suffice since some legal documents say at financial institutions only require an initial and in any case the person who initialled it can be contacted to provide his full signature and confirmation that the 6 months after the sale of the automobile can be used.