-Don't think DOL will accept the wage your employer/lawyer themselves got as the prevail wage for PERM.
-Because the PERM position is the one after you get the green card, your current salary does not matter to your approved PERM/I-140. So, you may not need to do anything to mess it up. If your employer file a new PERM and I-140 for the same position as the one for your approved I-140, your employer may need to withdraw the approved I-140 and that probably make you to take more time to get the green card.
If your employer can not pay you the prevail salary after you get the green card, you may just find a new job and move on after you get the green card.