Brown contended that Green owed him $6,000. Green denied that he owed Brown anything. Tired of the dispute, Green eventually signed a promissory note by which he promised to pay Brown $5,000 in settlement of their dispute. In an action by Brown against Green on the promissory note, which of the following, if true, would afford Green the best defense?
Although Brown honestly believed that $6,000 was owed by Green, Green knew that it was not owed.  Although Brown knew that the debt was not owed, Green honestly was in doubt as to whether it was owed.  The original claim was based on an oral agreement, which the Statute of Frauds required to be in writing.  The original claim was an action on a contract, which was barred by the applicable statute of limitations