Modric is a target of Chelsea, who have seen bids of £22 million and £27 million rejected by Tottenham, who, at the start of the summer, were adamant that the Croatian would not be sold.
However, Redknapp this week admitted the sale of Modric would fund a number of recruitments, thus improving his squad numbers for the new 2011-12 season as the club compete on several fronts including the Europa League.
Ferguson said: "Tottenham have said they're never going to sell him. That's what Daniel Levy has said.
"But then you've a feeling if someone comes in with the right offer he will be sold, and that's difficult for Harry, to work out Daniel Levy's thinking.
"I know Harry wants to bring in a couple of players, no doubt about that, because what he has achieved the last couple of seasons has been brilliant. So it has been difficult for Harry getting through the summer not quite sure what's going to happen.
"It could yet happen before the window closes he gets two or three players in, and I think he's hoping for that. I listened to him on the radio saying he could get four players in for the money. That's what he is thinking.
"In other words, giving him a bigger squad to compete in all the tournaments Spurs are in, and to challenge for the Premier League. So he was looking at improving his options."
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