Since I'm another Jeff, I will second the other Jeff's post.  You have to have a little trust in your clients.  If you don't trust them, do you really want to be doing their returns?  I have never held a client's return hostage for payment.  Maybe it's the anvils talking, but once I'm done with the return the client picks it up and pays me whenever he or she likes.  Some clients pay on the spot, some pay in a few days, and some pay in a few decades, but for the most part they pay.  Every once in a blue moon I write off an outstanding account but that is pretty rare.