Is seeing advice against marrying a girl after performing Istikharah a result of the Istikharah or an act of Satan?

Interpreting dreams is not our specialty, and we have previously clarified that a dream after istikhara (guidance-seeking prayer) is not to be relied upon or used as a basis for a ruling. The good outcome after istikhara is what actually happens, whether it's marriage or not. Acting upon istikhara does not depend on a dream, nor is it a condition for the validity of istikhara that the supplicant sees a dream afterwards. The supplicant, after their sincere istikhara, will only do what is good for them, and they should not base any practical ruling on any dream.