Seems strange that anyone would agree not to raise more funds or negotiate with other parties for their entire remaining runway.
I mean, it puts you in the situation that you either sell to that company or you go out of business. They could offer you a fraction of the amounts they were talking about before you signed, and you'd have no alternative.
Hell, even asking you to agree to such unbalanced terms seems like a sign they're planning to pull a fast one on you and they don't intend to negotiate in good faith.
What kind of chump would agree to shoot themselves in the foot like that, unless they'd exhausted every alternative?