terminated without cause shortly before sale of company

What is the name of your state? Texas

I was recently terminated without cause from a start-up company. I was given a small severance in return for signing a severance letter which basically surrenders all of my rights. I also had to abandon a large number of stock options. Less than 60 days after my termination, the company has nearly completed a sale of itself to a much larger company. I would be quite willing to return any severance in exchange for the abandoned stock rights. Do I have any rights in this area given the short interval between my termination and the sale of the company?

You'll need to show the separation/waiver agreement you signed to an attorney to get an answer to your question. If the document was properly constructed and you did indeed waive your right to pursue any legal action, then it's a done deal. It all depends upon the exact wording of the document however.

P.S. Your employer did not need "cause" to terminate you unless an employment contract was in place with terms to the contrary. In my experience, it's not uncommon for an organization whose CEO is contemplating selling it to reduce S&A expenses in order to make the "bottom line" look more attractive to prospective buyers.