do not worry, salary is not the key!

1. what the RFE is about?
2. does it only require Employment Verification letter?
3. when did you move?
4. how many days they give you to response?

Regarding the salary, it is not a big deal:
Read the policy here: http://www.myvisajobs.com/Tools/default.htm#Visa_Policy

Answer: No. As noted above the relevant inquiry is if the new position is the same or similar occupational classification to the alien’s I-140 employment. A difference in the wage offered on the approved labor certification, initial I-140 and the new employment cannot be used as a basis of a denial. However, a substantial discrepancy between the previous and the new wage may be taken into consideration as a factor in determining if the new employment is “same or similar.”