This is something that has never been confirmed by the creator of Death Note, who has said "readers should come to their own conclusions". Near stated that Mello acted on his own and that it was pure chance that Mikami ended up taking out the real notebook. However, Matsuda believes that Near manipulated events so that Mello would make a move in order to see if the notebook they had forged was the real one or if Light had instructed Mikami to keep it in hiding until the day of the meeting. I personally believe that Near may have manipulated Mello but it is also hinted at that Mello knew what Near was trying to do and allowed himself to become a sacrifical pawn in order to bring down Light. But just like the creator said, "Death Note is about readers coming to their own conclusions"