Highest oxidation state of manganese in fluoride is +4(MnF4) but highest oxidation state in oxides is +7(Mn2O7) because

1. Fluorine is more electronegative than oxygen
2. Fluorine does not possess d orbitals
3. Fluorine stabilises lower oxidation state
4. In covalent compounds, fluorine can form single bond only while oxygen forms double bond

Highest oxidation state of manganesein fluoride is +4(MnF4) but highest oxidation state in oxides is +7(Mn2O7). The reasonis that in covalent compounds fluorine can form single bond while oxygen forms double bond.