1. Your employer has to pay the salary stated on the approved LC, upon approval of your I-485. Your employer should be willing to write a letter to INS stating that they will pay the salary stated on the LC upon approval of the GC.
I was in the similar situation, and my employer was not willing to write the letter. Hence I had to restart the whole process. Luckily my attorney was able to find an approved labor and used it as a substitute LC to refile my I-140 which got approved without any problems.
Good Luck!!