parmpal
You did not mention if you received RFE at I-140 stage or I-485 stage. My understanding is, INS would never ask the employer at I-485 stage for proof of ability to pay wages as filed in the LC. That part is done at the I-140 stage itself. If the INS has send RFE asking for employer letter stating proferred wages, that means, it is asking for recent dated letter from the employer stating that it 'will' pay those wages as mentioned in the LC.
So it is not about proving the 'ability' to pay wages but the 'intention' to pay wages. I hope you understand the difference. All you need to do is ask your employer to provide a new letter again stating your position, duration of employment (optional), intention to continue hiring you and the willingness to pay the wages as per the terms and conditions specified in the labor, after you get the GC.