1. If you worked for the employer for six months AFTER getting your green card, then you can quit and go work somewhere else. If you do not have your green card, then the I-140 from the soon-to-be-ex employer will no longer do you any good.
2. Not that I have heard.
Andrew M. Bramante, Rosner Partners, 216-771-5588. Free telephone consultation. You should always consult with an experienced immigration attorney to make certain that the advice you received is appropriate for your particular immigration case.
Not only can they, any prudent employer would; We live in employment at will country; You are free to quit, they are free to cancel your I-140
NYC EXPERIENCED IMMIGRATION ATTORNEYS www.myattorneyusa.com; email: email@example.com; Phone: (866) 456-8654; Fax: 212-964-0440; Cell: 212-202-0325. The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter.
1) Yes, employer can and will likely withdraw their petition if you quit. You are able to port if I-140 is approved and I-485 is pending for over 180 days and new job is in "same or similar classification" unless USCIS revokes the i-140. Usually USCIS revokes upon finding of fraud.
This response is general in nature and cannot be construed as legal advice, given that not enough facts are known. Any comments offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship. If you would like additional information based on this response, please contact my office at 510 657 7665 or 415 902 0832 to schedule a consultation.