See clip from Attorney Rajiv S. Khanna's conference call video that addresses this question.
It is perfectly legal for an employer to process the green card for you as long as they have the good faith intention and you have the good faith intention of joining them. However once I-140 is approved and I-485 through a future employer has been pending 180 days you’re not required to join them. You could join anyone with same or similar job anywhere in the United States. So AC21 protects not only jobs you already have in hand it also protects future jobs. As long as the intention is honest basically that is based upon your statements and that’s all that matters.
Now you are protected by AC21 job portability or green card portability you can take any job anywhere with any employer as long as it is similar to the job described by your employer B when they file your green card.
Question: During I-1485 adjudication if I get EVL RFE and I decide to respond to it using another job offer from employer C, then - How do I prove that I certainly had intention to join employer B at the time of filing I-140 and I-1485?
How to prove that my employer B also had intentions to hire me on a permanent Job offer at the time of filing I-140 and I-485? If I cannot join them during I-485 adjudication.........can I use another job.
And the answer is - Yes you can and you don’t have to prove that this offer was extended prior to I-485 adjudication. That too can be a future job offer and the joining date does not really have to be specified all that needs to be specified is that is it is their intention to offer you the job and accommodate you either before or promptly after the green card is approved.
So you have all the options open if you want to respond using a job offer from the future employer (employer B/employer A) all of them are open to you and ability to pay RFE is usually only relevant to the employer who filed your green card. So if I use AC21 and go from employer X to employer Y USCIS is not going to question employer Y’s ability to pay. But if you stay with employer X they can question the ability to pay all the way till you actually get the green card.