Inelastic neutron scattering was used to study a quantum S=1/2 antiferromagnetic Heisenberg system- Bis(2-amino-5-fluoropyridinium) Tetrachlorocuprate(II). The magnetic excitation spectrum is shown to be dominated by long-lived excitations with an energy gap of Δ=1.07(3) meV. The measured dispersion relation is consistent with a simple two-dimensional square lattice of weakly coupled spin dimers. Comparing the data to a random phase approximation treatment of this model gives the intradimer and interdimer exchange constants J =1.45(2) meV and J′ =0.31(3) meV, respectively.