Does anyone have similar experience

on RFE fee? I paid off the lawyer fee stated in our contract after I got the GC. My wife's I-485 is still pending. My lawyer asks for $500 more for my RFE. In our contract it says "we will charge an additional fee if a significant amount of additional work is needed". But they did not ask me to pay the fee ahead of time as they did for my I-140. I wonder how the folks deal with RFE fees? Thank you!