On January 2, 2021, seventeen-year-old Martin paid $2,000 for a used motorboat to use in his fishing business, after Dealer’s fraudulent misrepresentation of the condition of the boat. Martin signed an installment contract for $1,500, and gave Dealer the following instrument as down payment: 

Dated: _________ 2021
I promise to pay to the order of Dealer, six months after date, the sum of $500 without interest. This is given as a down payment on an installment contract for a motorboat. (signed) Martin Dealer, on July 1, sold his business to Henry and included this note in the transaction. Dealer indorsed the note in blank and handed it to Henry, who left the note in his office safe. On July 10, Sharpie, an employee of Henry, without authority, stole the note and sold it to Bert for $300, indorsing the note “Sharpie.” At the time, in Bert’s presence, Sharpie filled in the date on the note as February 2, 2021. Bert demanded payment from Martin, who refused to pay. Explain what Bert’s rights are against Martin.