Is rejecting a job after performing Istikhara for it considered a violation of Istikhara, and does that require expiation or repentance?

The Istikhara prayer is legislated when a person is about to embark on a matter. If the matter becomes easy for him, then it is good. If it does not become easy or his resolve turns away from it, then that is an indication that it is not good for him.

Istikhara is recommended for supererogatory acts (nawafil) and permissible matters (mubahat), but not for obligatory acts (wajibat) or disliked acts (makruhat). Its time is before one resolves to do or abandon the action.

If the resolve of the one performing Istikhara turns away from the matter, then that is an indication that it is not good for him.