The I-140 is already approved. The standard 485 packet does not require an employer letter although you may want to include one stating that they intend to permanently employ you. The attorney who files the 485 will work out the details of the letter.
Your employer needs to issue you an employment confirmation letter to be filed at the same time as the I-485.
That letter has to have the same job title and description as in the PERM application. The letter has to renew the job offer and attest to the fact that it is permanent and full time at the same salary level as promised in the PERM application.
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Your employer needs to provide you a letter that confirms their continuing intention to employ you on a permanent basis for the position described in the Certified Form 9089 PERM application under the salary offered in the PERM application.
The H1B job description is Irrelevant for your I-485 process.