"You can absolutely guarantee that we're not going to give Ruud away," Ferguson was quoted as saying on United's official Web site on Tuesday during their tour of South Africa.
Ferguson said last week that the Dutch World Cup striker had asked to leave United and there was interest from Real Madrid and Bayern Munich.
Van Nistelrooy had a disappointing end to the 2005/06 season in which he was dropped by Ferguson.
"There's a possibility that Ruud will go," Ferguson said. "There have been bits of interest in him, but no one at this moment has reached our asking price.
"It's no surprise, really, that he asked to go, but when you see that Andriy Shevchenko went for 30 million pounds, he's a similar age to Ruud and their record in Europe is almost identical."
Shevchenko joined Premier League champions Chelsea from AC Milan in May but the fee was not disclosed by the club.