Working during Shiva due to loss for others

A person has a small independent accounting business, and his brother passed away, and now he is sitting Shiva. He has many reports he needs to submit, and if he does not submit them on time, the clients will be fined. Is he allowed to work during his mourning days?

Since if he does not submit the reports on time, others will be fined, this is considered a work of loss prevention, and therefore he can perform this work through others. If it is not possible to do it through others, he can be lenient by himself in private after the first three days. For Ashkenazim, it is permissible to be lenient in private within his home in case of significant loss within the first three days, while for Sephardim, it is forbidden to do it by himself even in case of significant loss within the first three days.

Gemara Moed Katan, page 11b: "Rabban Shimon ben Gamliel says: if his olives are overturned and there is no craftsman but him, his jug for the body and there is no craftsman but him, his flax to be raised from the solution, and his wool to be raised from the cauldron and there is no craftsman but him - let him do it in private." And earlier it is said: "Rav Shisha, son of Rav Idi, said: this implies that things permitted on a festival are forbidden during mourning days." The opinions of the early rabbis are divided on whether to rule like Rabban Shimon ben Gamliel or not, since Rav Shisha said it is forbidden during mourning even in case of loss. The opinion of Ramban and Rosh is that it is permitted by oneself in case of loss, however, Rambam and Rif disagree and hold it is forbidden. However, after the first three days, it is permitted to all.

The Shulchan Aruch, Siman 380, paragraph 4, prohibits by oneself in case of loss. And the Rema in paragraph 5 permits in case of significant loss within the first three days and without significant loss after three days.