All Perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as that of his family and all his companions.
The act of maintaining the records of interest based dealings falls under the ruling of writing down Ribaa (interest and/or usury) whose doer was cursed by the Prophet . Moreover, it might also come under the ruling of witnessing it while the dealing is being accomplished. As if the interest is charged spontaneously, as practiced in the case of the delayed payments, an accountant in this case calculates the amount of interest for the delayed payments and hence he commits the prohibition of writing and witnessing the usurious dealing.
Imaam An-Nawawi wrote explaining the above narration about Ribaa: "The narration clearly prohibits writing down and witnessing the usury contract of those who deal with it. It also prohibits any cooperation in evil."
Therefore, you know by now that it is forbidden for you to work in the mentioned field, and therefore you have to sincerely repent. Furthermore, the money you have earned from this work is forbidden. So, you have to get rid of that money by spending it on the Muslim public welfare, like helping poor and needy persons, building Islamic charity schools and the like. For more benefit please refer to Fataawa 81375 and 81451.
Allaah Knows best.