The 19-year-old has become one of Europe’s most sought-after talents this season, after scoring eight goals for the Austrian club during the Champions League group stage.
United are understood to be interested in the Norway international, along with Premier League rivals Liverpool and other European giants like Borussia Dortmund and Juventus.
But Freund warned the youngster against a move away in January, telling the Mirror: “It’s too fast for Erling.”
Salzburg have a strong reputation for developing young players, having seen the likes of Liverpool stars Naby Keita and Sadio Mane come through their ranks in recent years.
“I think it makes it easier [to sign players] for the future, if you have a lot of good examples, to bring a player here,” Freund continued.
"But sure this is a big challenge because the standard now from the team is more than four or five years ago.
"For example, five months ago, nine players leave us. Seven players normally in the starting XI, so it's a really big change for us.
"And in the end it was good that we prepared in winter...signing Antoine Bernede from Paris Saint-Germain and Erling Haaland .
“In the summer we only signed two new players, nine players leave us and I think this is an important thing, that we always have good players in the pipeline, and that they are ready to make the next steps when the outstanding players leave.
“This year it's a little bit strange because everything is going so fast. Also there’s interest in a lot of other players...Hee Chan Hwang, people seeing him play.
"Normally there's a healthy increase when they're one and a half or two years here in the first team.
"Erling has only been here five months in the team, sure he's outstanding… but, it’s football."
Freund also underlined that the Austrian champions have no intention of changing the way they operate any time soon.
“We have a lot of income and a lot of good transfer incomes,” he said.
"A lot of clubs change their strategy, giving more money to players or older players and we don't change our strategy.
"We are only focused on young players, highly talented players, and this is very important that we don't change. We are crystal clear on which players we want and also which coaches we want.
“We don't sign big name coaches, Marco Rose was a youth team coach, Jesse Marsch is first coach from America, is not a big name in Europe.
"We want to find players and coaches and our people in our club are convinced in our way. I think this is the most important decision that we go this way, 100 per cent.”
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