Well, I would talk to an employment lawyer instead of people on the internet to get an extremely clear picture of what your options are, but I'm also someone who is on a first name basis with my employment lawyer to the point that we exchange recipes.
As for dealing with the company, you are their top performer and they are trying to take away your client. Personally, I would probably be very firm with them that this is unacceptable. This doesn't mean causing a scene or being unreasonable, just being firm in your expectations of how you will be treated and compensated according to your established value to the company.
If they terminate you over that, would that not be an ideal case for demanding severance?
Overall, I'm not sure there's a "right" answer here, at least not with how vague the information we have is. If it were me it would depend on the job, the industry, my income, my spouse's income, our expenses, our savings, our emergency fund, the outlook for buying a business, the outlook for the potential performance of that business, etc, etc, etc.
There are a lot of factors that would personally make me more or less conservative on this front, but I'm not in your shoes, so can't tell what I would do if I were you.
