The Croatia playmaker has been the subject of transfer speculation for the second consecutive summer, with Real Madrid reportedly his likeliest destination.
Chelsea's interest was rebuffed before last season and Modric went on to play a starring role for Spurs under Redknapp's management.
But Redknapp warned the 26-year-old's excellence came at a price, with a promise to clear the way for a move away this year.
Real assistant coach Aitor Karanka recently praised the player in the Spanish press and Redknapp told the Sun: "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 (Daniel Levy) said to him last year `Give us one more season and if Real come in, I won't stand in your way'."