Ask HN: Got acquired now have some problems

Hello, Im one of first employees and still working in the company. We was acquired by one of NASDAQ companies for quite good money 2 years ago with kind of standart scheme like: Some money right now on the table and for the rest you have an option.

I own less then 1% of the company and never seen any papers except some documents regards first money trance and some SEC fillings regards our deal where was some notes regards that: company A acquired 80% of the company B, remaining 20% we have in option.

1 year ago i executed half of my stocks with option rights and got a money everything was just fine.

Now im trying to leave the company and sell my remaining stocks but the company offering me 3 times less the money and telling me sell or keep this stocks to yourself.

What can i do?