A screenwriter is a writer who performs the skill of screenwriting, writing screenplays on which mass media such as films, television programs, comics or video games are based.

As you have asked about the Screen writer salary agreement, check below for the information
Bottom salaries for union screenwriters are fixed by the Writers Guild of America.
Non-union screenwriters may write for free; a recognized screenwriter may write for millions of dollars.

Guarantee:
Factually, the money the writer is certain to receive, whether the script is produced or not. This amount is usually divided into steps with payments and due dates.
For example, a "three step deal" might include:

Step One:

First Draft Commencement (50% paid upon Commencement)
First Draft Delivery (50% paid upon Completion)

Step Two:

First Rewrite Commencement (50% paid upon Commencement)
First Rewrite Completion (50% paid upon Completion)

Step Three:

Polish Commencement (50% paid upon Commencement)
Polish Completion (50% paid upon Completion)[1]

The guaranteed money is sometimes referred to as the "front-end."

Optional Steps: The deal may often define optional steps that the studio can trigger at their discretion. For example:

Step Four:

Second Rewrite Commencement (50% paid upon Commencement)
Second Rewrite Completion (50% paid upon Completion)

Step Five:

Second Polish Commencement (50% paid upon Commencement)
Second Polish Completion (50% paid upon Completion)