I suggest that you get a good attorney. Your case can be approached in two ways. I am assuming you are still on H1. If you are on EAD then option 2 is the only choice.
1. Future employment: In this case, you should join company A after you get your green card. In this case, you clearly state that you are working for company B and will join company A after getting GC. You give letters from company B.
2. AC -21 case: If you do not intend at this point to work for company A after getting your GC, then you should invoke AC-21. This is perfectly legal. The law clearly states that there is no need for the applicant to have ever worked for the petitioning employer. But, the job offer should be bona fide at the time of 140 approval. In this case also you should send letter from company B and state that you are invoking AC-21. Your new job description should be as close as possible to the one on the labor.
Make sure you get a good legal help on this. If you are using AC-21, you need to convince USCIS that the job existed with compnay A and you had good intentions of joining company A, when your 140 was approved; but now you are using AC-21, for some reasons.
If you need to refer to specific law on AC-21 in your case, then you need to find out a memo that came out in August 2003. I believe it was from Yates. I have pasted part of the analysis of that memo here from another immigration site. To search for this analysis, google "BCIS Memo on I-485 Portability After I-140 Revocation". Choose the first hit.
Requirement of Bona Fide Job When I-140 Approved
The Memo requires that the offer of employment must have been bona fide and the employer's intent to employ the foreign national upon approval of the adjustment application must have existed at the time the I-140 was approved. This requirement is discussed under, "Analysis and Conclusion of this Memo," below.
No Need to Work for Sponsoring Employer to Use AC21
The Memo recognizes the fundamental fact that the entire permanent residency application process is for a future job offer and, accordingly, the I-485 applicant need not be working, or need ever to have worked, for the original sponsoring I-140 employer in order to benefit from AC21 portability. This is helpful and has been discussed below, under, "Analysis and Conclusion."
Sahithi said:
To be eligible for AC21. If I undestood correctly, one needs to wait till I-140 is cleared and 180 days pending I-485.
In my case I never worked for compnay A. But they contiued my Green card.
I was always on compnay B.
Now both the companies arte ready in my favor to provide whatever information required.
I don't want to confuse USCIS providing too much documentation.
Please suggest me a feasible solution.
Is providing company B information is good or company A?
I can provide company A but I cannot provide paychecks. Ofcourse the RFE did not ask for any paychecks.
RFE:
On company letterhead provide date of hire, remuneration, job duties, continued employment
Thanks in advance for your valuable suggestion
Regards