Generally, Employers could withdraw LC, I140 while USCIS could t

fraud, misrepresentation of a material facts were found on the GC holder. (i.e, an asylum GC holder if found freely go in and out the country he sought asylum from, GC might be taken back). An investigation would likely be called before GC was taken back.

Based on the info you provided here, it is very hard to judge how much threat that your previous employer could pose on your GC.

Highly unlikely ur old employer would request withdraw without providing certain evidence to USCIS that you obtained the GC "illegally (fraud, misrep, or other).

If they did make the request, the risk for them is equal to the risk you would face, false accusation,immigration fraud,black mailing etc.

This is a sword with double edges.

U had worked for them for two years, there were a lot of things you would know about this law firm. If they want $100,000 from u, less likely it is that clean in its other business dealings. Threat to threat works better if you know that kind of evidence they have on you and what you would have on them.

Highly unlikely a law firm would want USCIS investigator or US attorney jump on them for unethical dealings. U may lose a GC, they may lose the reputation and the license to practice and a lot of business.

good luck

Diclaimer: FYI only. Not a legal advise