  We consider the interaction pressure acting on the surface of a dielectric
sphere enclosed within a magnetodielectric cavity. We determine the sign of
this quantity regardless of the geometry of the cavity for systems at thermal
equilibrium, extending the Dzyaloshinskii-Lifshitz-Pitaevskii result for
homogeneous slabs. As in previous theorems regarding Casimir-Lifshitz forces,
the result is based on the scattering formalism. In this case the proof follows
from the variable phase approach of electromagnetic scattering. With this, we
present configurations in which both the interaction and the self-energy
contribution to the pressure tend to expand the sphere.
