What is the ruling on a lawyer's work in filing lawsuits against bank clients, demanding the principal amount of the debt and its legal or conventional interest, and is it permissible to refuse that work despite having an annual contract with the bank?

The fundamental principle is that the legal profession (advocacy) is permissible, unless the lawyer undertakes a false or unlawful cause. Thus, pleading for usurious interest is invalid, because it is prohibited usury, and it contradicts the command of Allah and His Messenger concerning the prohibition of usury. Consequently, a law firm is not permitted to litigate in usurious cases, even if there is a contract obligating it to do so, for contracts that violate the law of Allah are null and void. Similarly, the firm is absolutely not permitted to contract with usurious banks, due to the assistance this entails in sin and transgression.