Is it permissible for a translator who has signed a contract stating that the work should not be subcontracted to another party, to seek the help of colleagues in the translation, provided that he himself reviews the work and delivers it to the client, without informing the client of this?

If a person is hired for a job and it is not stipulated that he must perform it himself, then he may delegate another to act on his behalf, even if it is for less than his own wage, unless the work is such that its quality varies depending on who performs it, like the quality of calligraphy, for instance. However, if the hirer stipulated that he must perform the work himself, then he is obligated to adhere to this condition. Otherwise, it is not permissible for him to delegate, and such an act would be considered a betrayal.