Dispelling the rumour that you get locked in, you do not get locked in once you give him an item to upgrade.
First you talk to him, and inquire about sets. Next time you talk to him it should say "This is the one" once you picked a set/item for upgrade.
http://img301.imageshack.us/img301/9758/armor1gt9.jpg
Trade him the peice :
http://img301.imageshack.us/img301/9163/armor2ib9.jpg
And you are not locked in. You can retreive the peice, and the best thing is you can retreive single peices of it. He then becomes an armor storage NPC for single peices, and wont take any fee untill you have it all.
http://img84.imageshack.us/img84/8945/armor4fw2.jpg
I think however you'er locked into a "set" - like you cant give him skadi and morrigan, even though i'm a greedy lootwhore I don't have 2 peices of different sets so I can't test if you're locked into sets, since you're definitely not locked into peices.