This sounds like a significant business dispute covered by specific contracts and regulations. You should get a lawyer. If you do not have a lawyer and have a board of directors, funders, or the like, this is a good resource to use to get a lawyer.

Anyone advising specific actions to take in this thread that aren't "get a lawyer, preferably an expert in this field, and follow their advice" is doing you a disservice. This is almost certainly something you can't DIY. If the monetary amounts are too small to be worth a lawyer, you have very limited options.

Get. A. Lawyer.