  Charge transfer (CT) insulators are the parent phase of a large group of
today's unconventional high-temperature superconductors. Here we study
experimentally and theoretically the interband excitations of the CT insulator
silver fluoride AgF$_2$, which has been proposed as an excellent analogue of
oxocuprates. Optical conductivity and resonant inelastic X-ray scattering
(RIXS) on AgF$_2$ polycrystalline sample show a close similarity with that
measured on undoped La$_2$CuO$_4$. While the former shows a CT gap $\sim$3.4
eV, larger than in the cuprate, $dd$ excitations are nearly at the same energy
in the two materials. DFT and exact diagonalization cluster computations of the
multiplet spectra show that AgF$_2$ is more covalent than the cuprate, in spite
of the larger fundamental gap. Furthermore, we show that AgF$_2$ is at the
verge of a charge transfer instability. The overall resemblance of our data on
AgF$_2$ to those published previously on La$_2$CuO$_4$ suggests that the
underlying CT insulator physics is the same, while AgF$_2$ could also benefit
from a proximity to a charge density wave phase as in BaBiO$_3$. Therefore, our
work provides a compelling support to the future use of fluoroargentates for
materials' engineering of novel high-temperature superconductors.
