  The Mott insulator $\kappa$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$ forms a
highly-frustrated triangular lattice of $S=1/2$ dimers with a possible
quantum-spin-liquid state. Our experimental and numerical studies reveal the
emergence of a slight charge imbalance between crystallographically
inequivalent sites, relaxor dielectric response and hopping dc transport. In a
broader perspective we conclude that the universal properties of
strongly-correlated charge-transfer salts with spin liquid state are an
anion-supported valence band and cyanide-induced quasi-degenerate electronic
configurations in the relaxed state. The generic low-energy excitations are
caused by charged domain walls rather than by fluctuating electric dipoles.
They give rise to glassy dynamics characteristic of dimerized Mott insulators,
including the sibling compound $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$.
