"If your form I-485 is related to an employment-based visa petition (Form I-140), you must a letter on the letterhead of the petitioning employer which confirms that the job on which the visa petition is based is still available to you. The letter must also state the salary that will be paid"
Form I-140 (EB1A) petition is not employment-based. So I wonder whether I need to provide. But not sure, and come here to inquire, thanks for your help. Do you have any reference web-site or something? I hope to confirm.