With progressive increase in atomic number, the reduction potential of halogens decreases, thus oxidising power also decreases. Hence, a halogen with lower atomic number will oxidise the halide ion of higher atomic number and therefore, will liberate them from their salt solution.
Hence, the reaction
$$
\mathrm{Cl}_2+2 \mathrm{~F}^{-} \longrightarrow 2 \mathrm{Cl}^{-}+\mathrm{F}_2
$$
is not possible.
,