Sporting want £34.1m (40m euros) for the 28-year-old, which the Foxes consider unrealistic.
Slimani, who is keen on a Premier League move, has been at Sporting since 2013 and has scored 31 goals in 46 games for the club.
The striker made four appearances for Algeria at the 2014 World Cup in Brazil, scoring twice.
Premier League champions Leicester City are in talks with Sporting Lisbon to sign Algeria forward Islam Slimani.