Don't agree to anything. That is all you have to do. When the company tries to raise money later, they will be asked about any other people who have an interest. At that point your company will need to address your issues and they will have money/incentive to do it. You don't need to be mean spirited, just say "Okay, I have my 40% and let me know if in the future you want to buy me out." And be fair.

Don't walk away, don't sign anything that isn't fair to you, don't be mad, just be patient.