What recourse do I have if my employer is unwilling to honor a signed sales compensation plan at the time payout is due?

I am a Sales Manager. My total compensation consists of a base salary and an incentive piece, paid biannually. The incentive piece is a substantial part of my income. I entered into the sales incentive plan contract when I signed the plan document in May of 2015. The President of my company also signed the document, and it was submitted to HR. My company's HR Director was the individual who drafted the plan and presented it to all parties involved for review and signature. Incentive payment was made at mid year, but year-end payment is currently being withheld because my employer is now claiming that a mistake was made in the incentive calculation as indicated on the plan; and that the plan does not capture the company's "intent". They have since revised the sales incentive calculation (significantly less money) and have presented it to me for signature. I have refused to sign the change. They state that they will not pay me any part of the incentive until I sign the revision.