Single crystals of ferric borate (FeBO₃), annealed in vacuum so as to provide for the oxygen deficiency and the corresponding excess Fe ²⁺ ion concentration, have been studied by ⁵⁷Fe NMR. The NMR signal intensity and the transverse nuclear relaxation rate in oxygen-deficient ferric borate are significantly increased compared to those in the initial state. The annealed samples also exhibit a giant shift of the resonance frequency under the action of light, which is several times as large as the analogous shift in samples with the equilibrium concentration of Fe ²⁺ ions. The obtained experimental data confirm the model of photomagnetic phenomena in antiferromagnetic materials that is based on the notion of Fe²⁺-containing complexes playing the role of photosensitive centers.