Talk to an attorney about this. This is serious, and requires legal advice from a professional. Expect to pay for a few hours of counsel. They'll have to look at the documentation and know a lot more in detail about your situation, promises made, obligations, legal recourse, etc.
Question added to topic Legal Basics For Startups • September 12, 2007
What should I do when my investors only finance one third of the agreed upon cash and say they are out of funds even though we have a signed agreement?
I entered into an agreement for a software project with two other guys who agreed in writing to finance the project for a certain amount. They have sent one third of the money and say they have no more now. What should I do? I have committed my company's resources to this project and was counting on that funding.