Unclear
I wish I could answer this question for sure. You certainly are not required to work for the sponsoring employer (or any employer at all for that matter) until the GC is approved. The question then is one of whether AC21 portability will apply, when, and under what circumstances. It seems to me that AC21 certainly (or at least certainly if AC21 is to have any meaning at all) applies if you work for the sponsoring employer for 180 days after the I-485 is filed (I don't know for sure if this clock begins to run only when the I-140 is approved if the I-140 and I-485 are filed concurrently or whether (as the language of AC21 seems to clearly indicate) the clock starts when the I-485 is filed regardless of whether the I-485 and I-140 are filed concurrently.