Afraid Employer Will try to screw me out of bonus earned

I am a grant writer for a non profit. I am paid hourly and also on a Grant Writer/Incentive Bonus plan, which I have a copy of. The terms are that I would be paid on a percentage 2%, 3% and 10%(with a cap of $5,000 per grant) depending on the grant amount. I had already received bonuses in the 2% range, but worked hard and we were approved for a grant that now put me in the 10% range and was over $50,000 so my bonus due would be $5,000. The terms of the agreement were that I would be paid on the first pay date after the funds were received by my organization. This condition was met last summer 2003. I was not paid because the organization has been experiencing problems making payroll etc...but my boss assured me and my supervisor that he knew I had $5,000 coming. A few weeks ago, my organization received over $100,000 from our City, and so I put in an invoice (which is not my job) to receive my money for the bonus. Just yesterday, my employer terminated me after I said something to him in defense of another employee. I then told him that was fine, but I want my $5,000 that he owes me for the grant. He did not respond. I have also contacted the President of our Board of Directors, who happens to be an attorney, to let him know I want my money. Mostly every employee in the organization knows that I get grant bonuses. Will he be able to deny my bonus? I am very worried, especially now that I am unemployed.