We formulate an exact constraint on the interacting one-particle Green function of a compensated two-sublatticemodel whose symmetry combines a quarter-turn in momentum space with a spin and sublattice transformation. For anequilibrium density matrix invariant under this operation, the full Green matrix obeys a covariance relation that exchanges thespin-resolved sublattice blocks at rotated momenta. The traced spectral functions consequently satisfy ��↑ (k, ��) = ��↓ (��k, ��),while the Brillouin-zone-integrated uniform spin polarization vanishes. The identity is independent of perturbative order anddoes not assume quasiparticles; it constrains, but does not by itself require, a nonzero momentum-resolved splitting. We realizethe symmetry in a two-sublattice Hubbard model with a compensated staggered source, derive its analytic noninteractingspectrum and first-order Hartree renormalization, and test the interacting spectral structure by Lanczos exact diagonalizationon an eight-site cluster. We further calculate the full nonlocal second-order Hubbard self-energy in real space, express itsretarded continuation as a three-spectral-function convolution, and verify its symmetry covariance and causal sign numerically.The results establish a symmetry-controlled route to interaction-renormalized altermagnetic spectral splitting in a compensated,source-pinned state.