A frum lawyer represents a frum client in taking a frum defendant to court in a case where according to the Torah the defendant would not have to pay anything but according to the secular law they have to pay.
Does the lawyer have a din of a moser so that the defendant would later have the option of being able to take either the claimant to a din torah or the lawyer to a din torah in order to recover the money they were forced to pay by the secular court?
Before saying anything else, it must be said that going to court is a severe prohibition, and the Shulchan Aruch classifies it as “raising one’s hand against the Torah of Moshe” – meaning, it is a statement that secular law is more just and worthy than Torah law.
In this specific case, the lawyer will not have the halachah of a moser, for this specific category does not generally apply to civil cases.
However, If the money is being taken from the defendant against Torah law, it follows that the defendant will be able to claim the money back from the plaintiff in beis din, and that the lawyer is “aiding and abetting” the plaintiff to do a grave aveirah of going to courts of law to extract undeserved moneys.
The halachos of the matter are ruled by the Shulchan Aruch and its commentaries, Choshen Mishpat siman 26.