Highly rated Norwegian striker Haaland signed a four-and-a-half-year deal with Dortmund, despite strong interest from United.
United, along with RB Leipzig and Juventus, were among the admirers of Haaland – who scored 28 goals in 22 appearances across all competitions for Salzburg this season.
Haaland revealed why he opted to join Dortmund, with the 19-year-old sensation saying: "They just went direct and said 'we need you upfront, we like your playing style and we want to have you here'.
"I liked how they spoke to me then and that's what triggered me [to sign]. I just felt that me and Dortmund was a good match."
Haaland joins a Dortmund side fourth in the Bundesliga and seven points behind leaders Leipzig after 17 games.
Lucien Favre's Dortmund do not return action until January 18 due to the midseason break.
"To slightly get to know people better, to train good with the team and we'll see if I am good enough I will play and we'll just have to see, step by step," Haaland said of his aims at Dortmund.
"Of course, I want to play and I know I have something to give to the team. That's my main goal now to have smaller goals and to reach them and I know it will be good."