Moniker.com do escrows, when they did it for a previous sale I was involved with both the cash and the site were held by them until all assets had been transferred accordingly. You may want to look into using them being that they can offer escrow for both parties assets, meaning the cash and the site.
Check out this page
http://www.moniker.com/aftermarket/domain-escrow.jsp
Good luck with everything and I hope the transition runs smoothly!
