forgivable promissory note / bonus

I was offered a $50k promissory note by my new employer (NC) payable in 3 years that we would use to help purchase a house. It was agreed upon (but not documented) that my mid-year bonus' would forgive the loan each year over the three years. In order to satisfy our mortgage loan underwriters, after the date of the promissory note, the employer wrote a letter stating the money was a incentive payment for services. The company has since altered their bonus structure and did not provide me with bonus' equal to 1/3 of the bonus amount. Therefore, when the note term ends, I will owe the majority of the note. What options do I have?