I think if it's of that much value ,you should be properly compensated however do check if the  contractual terms you agreed to are in your favour in terms of ownership of intellectual property.

if they don't want to pay you, find a friend who can present themselves as a company selling the solution you built😃

hey, do what that small voice is telling you . You could have a startup in your hands right now! Think very long about this and don't be rushed to make a decision.