I agree with atlantabhopali. Though GC is for future job, but USCIS generates RFE for paystubs/W2/EVL regularly. They never generate any RFE to know how much employer will pay after GC approval. It's a common sense that an employer does not pay an employee based on the condition that before getting GC the employee will be underpaid and then after GC approval his/her salary will be doubled. Current salary is certainly a good indication for your "future salary" (for your future job).
On the other hand, your good attorney (if he/she is good at all) can make a case pulrely based on the argument that your LC is for future employment. But I would not count on it.
Secondly, some employers bypass W2 and pay some amount in per-diem basis - just to avoid tax. Small amount it ok, but half of the salary is a problem. If USCIS catches that, it could be troubling for your employer. There is no reason that your GC will be denied on that basis, but it can get delayed for long long time.
Your best option is to negotiate with current employer and make a deal so that it can be good for both sides. If that does not work out, wait for another five months to cross 180 days mark and look for another job.
