Intermediate valent tri- and tetravalent manganese ions play an important role in LaMnO3-based systems. We consider a Mn impurity with five orbitals in cubic symmetry which hybridize with conduction electrons. The exchange interaction in the d shell maximizes the impurity spin. We study the valence of the Mn impurity as a function of the splitting of the eg to t2g orbitals in zero magnetic field and for the totally spin-polarized state. The lifting of the degeneracy of the eg levels due to a small quadrupolar field, related to the Mn-O bond length or a Jahn-Teller effect, is also investigated. Possible implications on the magnetoresistance are discussed.