Can employer FORCE me to postpone filing I-485?

I need some advice from the people on this board.

My labor certification was recently approved via PERM. My employer will soon be signing the paperwork to file the I-140 with INS. My contract with him expires in February 2007, and he wants me to continue working for him beyond that. I, however, have expressed my desire NOT to stay with him any longer than I need to. Obviously, I will need to stay until 6 months have passed from the filing of my I-485 for portability to kick in.

Because of this, he wants me to WAIT until January 2007 to file my I-485. This way he is assured that I will be working for him until June 2007. My question is, does he have any right to coerce me to wait until January 2007 to file the I-485?

I initially agreed to this delay in filing because I was under the impression that BOTH the I-140 and I-485 were to be filed by the employer, and that I-485 processing took about 6 months. Now that I have learned that the I-485 is to be filed by me and that I-485 processing can take more than a year, are there any downsides to me filing the I-485 earlier than January 2007 WITHOUT MY EMPLOYER KNOWING?

I realize that "honesty is the best policy" but the situation is truly untenable for me and I feel that he is purposely delaying the processing of my INS papers to keep me at his mercy. Another factor to consider is that the lawyer we will be using for the I-140 will be the same one who will file my I-485. Can I invoke attorney-client privilege with regards to the I-485 so that they cannot tell my employer that I filed it earlier than he wanted? For that matter, can I use a different lawyer to file the I-485 that the one who filed the I-140?

Any opinions and suggestions regarding this matter will be greatly appreciated. Thank you very much and good luck to all!