We consider a boundary optimal control problem for the acoustic wave equation with a final value cost criterion. A time-domain decomposition procedure for the corresponding optimality system is introduced, which leads to a sequence of uncoupled optimality systems of local-in-time optimal control problems. The process is inherently parallel and is suitable for real-time control applications. Convergence of the local solutions and controls to the global ones was established in an earlier publication. In this paper, a posteriori error estimates of the difference between the local solutions and the global one are obtained in terms of the mismatch of the nth iterates, or of successive iterates, across the time-domain break points.