Haaland, 19, scored both goals with one coming in each half, as Dortmund put six points between them and their opponents Leipzig to finish another season second to record champions Bayern Munich.
The Norway international was set up for the first goal by United States youngster Reyna, 17, who picked up the assist after passing the ball to Haaland in the box following a perfectly executed counterattack by Borussia Dortmund in the first half.
Haaland, who added his second goal only moments before the final whistle, later praised Reyna.
"I called him the American dream before and that's true. He is 17 years old and what he was doing on the pitch today was amazing," he said. "He has a huge future in front of him."
The 20-year-old defender had previously played in 27 matches for Bayern's reserves in the third league.