For something that big, I'd consider personally meeting with your client to do the exchange. Large banks in your area might have escrow services as well.
If that's out of the question, then consider Sedo, Afternic, or Escrow.com.
I've used AfterNIC on numerous occasions without trouble. The folks @ AfterNIC are very helpful and attentive, IMO.
I used escrow.com for my last domain sale and it went OK. I needed to call them so that a tech' could refresh their database after I had to change my email address, but that was the only glitch (minor.)