An austenite-martensite transition was observed in a 100-nm-thick Ni51.6Mn32.9Sn15.5 film by temperature-dependent resistivity and magnetization measurements, revealing a martensite starting temperature of M-S approximate to 260 K. The influence of the structural phase transition on the electronic structure and the magnetic properties was studied element specifically employing temperature-dependent x-ray-absorption spectroscopy and x-ray magnetic circular dichroism. In addition, density functional theory calculations have been performed to study the electronic and magnetic properties of both phases. It is shown that off-stoichiometric Ni-Mn-Sn alloys can exhibit a substantial magnetocrystalline anisotropy energy in the martensite phase. For Mn a change of the electronic structure and a strong increase of the ratio of orbital to spin magnetic moment m(l)/m(S) can be observed, whereas for Ni nearly no changes occur. Applying an external magnetic field of B = 3 T reverses the change of the electronic structure of Mn and reduces the ratio of m(l)/m(S) from 13.5 to approximate to 1 % indicating a field-induced reverse martensitic transition.