Is it permissible to secretly work in a company on behalf of another person, while receiving the wage in his name?

It is permissible for a woman to work in her husband's brother's job and receive the agreed-upon wage between them, provided that the entity for which her husband's brother works does not stipulate that he must perform the work himself. If the employer stipulates that the contractor must perform the work himself, then he is not allowed to seek help from others. However, if the employer does not stipulate this, then he is permitted to hire someone to perform the work, unless the work is intended to be performed by the contractor himself due to a distinctive quality he possesses.