My severance agreement does not include all the benefits previously agreed upon, do I have recourse?

Our company was merged. As part of the merger all employees were offered jobs at their current pay with the job description left TBD. If employees chose not accept these new job offers, they could elect severance to accept a severance package. My new job offer did not include a job location or job description, and to me, it was just too vague to accept in lieu of the severance package we were given details about. I gave my two weeks notice and received a separation agreement. My separation agreement does not include all the benefits we were told it would include. When I questioned HR, I was given a letter stating the additional benefits would be honored. The letter was dated, signed and written on letterhead. Is this letter legally binding or is it a trick to get me to sign the agreement without the additional benefits? I am extremely skeptical of this new organization based on my experiences so far.