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

i) Fluorine is more electronegative than oxygen.
ii) Fluorine does not possess d-orbitals.
iii) Fluorine stabilizes lower oxidation state.
iv) In covalent compounds, fluorine can form single bonds only while oxygen forms double bonds.