Modric has been the subject of intense transfer speculation this summer, with Premier League rivals Manchester United and Chelsea both said to be keen on signing the 25-year-old midfielder.
Last month, Chelsea saw a £22 million bid for the Croatian rejected by Tottenham chairman Daniel Levy, but it is reported the West London side are still hoping to seal a deal for the former Dinamo Zagreb star.
Levy has gone public with his insistence the player will not be sold, but some media outlets have suggested this week they would admit defeat in their battle to hold onto their star man, should a £35 million bid be tabled.
But Spurs boss Redknapp has insisted that an offer in that region would still not be enough to Modric leave White Hart Lane.
"If we were looking to sell him it wouldn't meet my valuation," Redknapp said. "I think he is worth an awful lot more money than that.
"Really we're not looking to sell Luka. We need to keep our best players so he is a player that we want to keep if possible.
"It is in the hands of the chairman of the football club [Levy]. It's not my decision at the end of the day.
"I obviously want to keep him as the coach but the chairman says he is not for sale so we'll have to wait and see.
"We're quite determined, if we can, to hang onto him but it is a difficult situation.
"It's a difficult one for Luka and it's a difficult one for Tottenham."
Spurs will continue their pre-season preparations by facing Orlando Priates in the final of the Vodacom Challenge at Ellis Park, Johannesburg on Saturday afternoon.
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