[maxklein]

Tell them that you want a handwritten promise of what you agreed upon verbally. No specific details, just a simple hand written promise at this moment, with a specific 3 or 4 week deadline to make this formal. If they say no, you tell them that they are acting in bad faith, and that you are going to stop work immediately.

An agreement is an agreement - if you have emails or chat messages or so on, that's already a contract.

They have a choice - give you something handwritten now or they lose their developer at the point where they are about to close a deal, which is a really bad time. They will tell you that this is the wrong time to apply pressure, but for you, this is exactly the right time. This is when they are most vulnerable because the deal could fall through, so this is your best chance to get this through.

When they have investment, how useful do you think you will be? They will just get rid of you and hire someone new.