Early experiments on collisional detachment of electrons from negative ions were interpreted in terms of a "local" or "static" complex potential. In the preceding paper we developed a more general close-coupling framework for describing such collisions. In this theory, the amplitude for finding the electron in the bound state C−1(t) satisfies an integro-differential equation. We define an "exact" or "dynamical" complex potential E(t) such that C−1(t)≡exp [−i ∫t0tE(t′)dt′ℏ]. An integral equation for E(t) is derived and solved. It is found that in slow collisions, the dynamical complex potential oscillates about the static complex potential. Approximate formulas for E(t) are given.