The former Spurs manager was in charge last year when the Croatia international asked for a move away from White Hart Lane, before being convinced to stay in north London.
Modrić has been relentlessly linked with a switch to the La Liga champions all summer and Redknapp hinted the playmaker could be set to leave club.
“I would not be sure Luka would start the season at Tottenham. The club would be happy to sell if they get the right money. The chairman said to him last year ‘Give us one more season and if Real come in, I won’t stand in your way’,” Redknapp told the Sun.
Ealier in the week, André Villas-Boas admitted he would have to assess Modrić’s mental state before deciding when he can bring the midfielder back into the Tottenham first-team squad.
“It’s difficult,” Villas-Boas said. “Obviously he has to pick up on his fitness but we’re talking about a player with tremendous talent.
“In a way, the most important thing is to analyse his situation as it stands at the moment.
“See how the player feels mentally to return and the fact that the club has not received any offers that are rated highly for the talent that he is.
“So I have to speak to the player and I think he will understand because he’s a good professional and one who has served Tottenham to the highest level.
“Hopefully we will get him back but if not we will continue to monitor his situation and improve on his fitness on an individual basis.”
Modrić has scored 17 goals in 159 appearances for Spurs after completing a £16.5m move to White Hart Lane in the summer of 2008.