To sort out this issue, you are best advised engaging the services of a trained Arbitrator which should bring the fairest outcome. A professional arbitrator will lead the process from an impartial standpoint and guide the negotiations to a fruitful conclusion. Having a skillful negotiator on board who sets the ground rules, commands the respect of both parties, is a skilled communicator, and concludes a final settlement agreeable to both parties
is what you need here.
You may also need the services of a solicitor and accountant to advise on specific aspects of the "settlement" but in my experience these professions do not usually shine in the finer art of negotiations, diplomacy, interpersonal
relationships and brokering deals.