The striker is attracting the interest of several clubs this summer after scoring 31 goals last season in all competitions.
West Brom have been heavily linked with the Algerian, while Everton are also rumoured to be monitoring the player.
However, according to France Football, Leicester are in the running too but they have had a £21m offer rejected.
Slimani did have a release-clause of £25m earlier this summer - but that expired on 15th June after no one activated it.
Now, Sporting are holding out for around £34m and will not budge unless someone stumps up that amount.