2. It is forbidden to act as an informant on your brother. One who does so loses his portion in the World to Come. To protect society, it is a mitzvah to eliminate an established informant who threatens the public or one who is presently threatening to inform .
3. However, one may direct a pursuer to his/her brother's money in order to save him/herself from physical danger.
4. Nevertheless, one who physically hands over his/her brother's money when forced to do so must reimburse the owner. If the pursuer is able to access the money without assistance, then one who hands it over is absolved from compensating the owner, as the act of "handing over" does not constitute an act of damage [Choshen Mishpat 388: 3, 4, 10, 11, 12, 15].
Application
An IRS accountant, or Internal Revenue Agent, has a wide range of duties. Reporting tax fraud is only a small part of the job. The chance of encountering a situation of having to incriminate a brother is minimal. As such, while applying for such a job is not a pious venue [Rav Shmuel Wosner] it is not forbidden [Rav Moshe Feinstein, Rav Shmuel Wosner].
Once on the job, if discovered, failure to properly report one's findings about his/her brother could cause a Chillul Hashem. The agent is compelled to be truthful [Igros Moshe Choshen Mishpat 1: 92, Shu"t Shevet Halevi 2:98].
The IRS generally has the ability to find out with or without the particular agent's work. As such, the agent would not be required to reimburse his brother, should the agent have to require his/her brother to pay more money.
However, an IRS Criminal Investigation agent's job is to search for evidence of criminal conduct. It would follow that were there reason to believe that the job would entail incriminating a brother, it would be forbidden to apply for such a job as well as to join a sting operation against community members. If one was compelled to take the job, then he would be permitted to carry out his duties if the community members halachically owed the government money because of dina demalchuta dina [Shu"t Shevet Halevi 2:98, see Be'er Hagolah C.M. 388 in Europe common practice was that the community board was appointed by the ruling government to ensure accurate taxation compliance. The minhag was to enforce the tax laws of the governing government].
