the sponsor has intent to employ the benificiary and the benificiary has the intent to engage in the employment upon 485 approval at all these time: when LC is files, when 140 is filed and approved, when 485 is filed and approved. so basically both parties have to maintain the intention to employ/to be employed all through the whole process. If USCIS determines at any of these times there was such a job or intent, they may deem it as fraud and can revoke it.
Now you get gc yesterday, and you resign today. How can you convince cis that you did have intent yesterday but you suddenly changed your mind today, if cis ever knows it? But if you keep work for them for a reasonable time period (no fixed rule but generally 6 mo is enough) and then resign. You can say when gc was approved you did have the intent but now my mind has changed.
of course if no one reports it probably you can get away with it but it is like a nightmare that can come back to bite you in the future. cis can revoke your gc if they determine it was based on fraud. so work for several months before you jump so you can sleep better in the nights.
公司故意說其實當初沒有什麽永久職位? the sponsor was engaged in fraud when they filed LC and 140 for you. they will get penalty. if the relationship get so bad that they are willing to bear the penalties (remember the excutive who filed LC/140 for you signed forms under penalties of purjery) to screw you, well, you'd better not to work for them in the first place.