All perfect praise be to Allah The Lord of the Worlds. May His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
It is forbidden for a Muslim to work in all that helps in eating up people`s wealth unjustly. The lawyer who accepts being assigned by the bank to argue cases of claiming usurious amounts from debtors is actually helping in sin and transgression and is involved in this forbidden transaction. The evidence on this is that Allah the Almighty Says {What means}: "O ye who believe! Profane not Allah’s monuments nor the Sacred Month nor the offerings nor the garlands, nor those repairing to the Sacred House, seeking the grace and pleasure of their Lord. But when ye have left the sacred territory, then go hunting (if ye will). And let not your hatred of a folk who (once) stopped your going to the inviolable place of worship seduce you to transgress; but help ye one another unto righteousness and pious duty. Help not one another unto sin and transgression, but keep your duty to Allah. Lo! Allah is severe in punishment." {Al-Mai`dah, 2}. Another evidence is that Jabir (RA) narrated: "Allah's Messenger (PBUH) cursed the one who accepts usury, the one who gives it, the one who records it and the two witnesses to it, saying: "They are all the same." [Reported by Muslim]. Annawawi commented on this Hadith: "Its meaning includes the prohibition of helping in sin." {Shareh Muslim, vol.11/pp.26}.
In conclusion, it is forbidden for you to accept being assigned to argue such cases unless the bank gives up all the usurious amounts it claims from debtors. As for your salary, it is a mixture of lawful and unlawful money. The lawful portion is what you take for defending people`s rights and the unlawful portion is what you take for arguing cases of claiming usurious amounts from debtors. It is incumbent upon you to cleanse your income from ill-gotten money and remember that Allah Says {What means}: "Thus when they fulfil their term appointed, either take them back on equitable terms or part with them on equitable terms; and take for witness two persons from among you, endued with justice, and establish the evidence (as) before God. Such is the admonition given to him who believes in God and the Last Day. And for those who fear God, He (ever) prepares a way out, And He provides for him from (sources) he never could imagine. And if any one puts his trust in God, sufficient is (God) for him. For God will surely accomplish his purpose: verily, for all things has God appointed a due proportion." [At-Talaq 2-3] And Allah The Almighty Knows Best.