Hello my name is ***** ***** I am delighted to answer your questions today. I have 35 years experience as an immigration lawyer.
My questions are -
Do I need to inform employer that I can file 485?
Yes, you should. You also need a copy of the fee receipt for the i-140 - form I-797
If I file 485 myself and any rfe comes for 140[NOT 485], will the employer get to know that I filed 485(As its concurrent)?
No, but why hide it from the employer. The it appears you are trying to deceive them and it makes you look bad.
Do I need an  approval notice to file 485 post approval of 140? Assuming I join their firm?
Can it be worked out with receipt.
I don't understand this.
Is offer letter enough to validate employment verification?
I don't understand this. You don't need any employment verification for the 485. The I-140 fee receipt is all you need.
What if the salary[15k lower than 130k] in offer letter and location is different from whats being shown in PERM application?
The PERM and I-140 are prospective in nature. That means that it is the offer for that position, at that location at that salary stated in the PERM and the I-140 upon approval of your adjustment of status. You do not have to be working in the position, at the prevailing wage in the job site stated in the PERM and I-140 until the adjustment of status is approved.
At some point you may receive or the employer may receive a RFE requesting a statement that it is their intention to employ you as per the PERM and I-140 when the adjustment of status is approved.
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