We report an Inelastic Neutron Scattering (INS) study of the fully deuterated molecular compound K₆[V₁₅^IVAs₆O₄₂] · 9D₂O (V₁₅). Due to geometrical frustration, the essential physics at low temperatures of the V₁₅ cluster containing 15 coupled V⁴⁺ (S = 1/2) is determined by three weakly coupled spin-(1/2) on a triangle. The INS spectra at low energy allow us to directly determine the effective exchange coupling J₀ = 0.211(2) meV within the triangle and the gap 2Δ = 0.035(2) meV between the two spin-(1/2) doublets of the ground state. Results are discussed in terms of deviations from trigonal symmetry and Dzyaloshinskii-Moriya (DM) interactions.