writing a screenplay for a novel

I am a writer working on a screenplay for a novel that came out in the 80s. The author is dead and the movie rights have been purchased, but no movie was ever made. Please correct me if I am wrong, but my understanding was that to recieve screenplay writing rights you had to have the author's permission. If I contact the estate before pitching the screenplay, is there any way their permission would help me legally down the road, or is the matter of the movie being made completely up to the company that bought the movie rights?

Also, are there any tips anyone has on trying to keep the screenplay rights exclusively to myself, assuming the project is optioned?

Thanks ahead of time, and I apologize for any inconsistencies this post has with the forum protocol, which I admittedly have not checked out.

There's no point in contacting the estate to obtain something that belongs to someone else.  They can't give you what isn't theirs.  If you want to present the treatment, you need to talk to whomever bought and holds the rights. 

I seem to recall reading somewhere that movie rights are sometimes purchased for a limited time and if a movie is not made by the expiration of that time, then it reverts back to the author (or estate in this case).

So, basically, you'll need to find out which one owns the rights and deal with one or the other.