But you can indeed blame him, because it isnt in his inventory yet. That's just it, if you can trust him to cast a lot, you can just as easily trust him to pass the item to the one who needs it.
Basically, whichever system you have, there is an equal chance someone is gonna rip you off. Those are the risks you take when partying with someone you dont know.
You're not seeing that there is, essentially, already a lot taking place. If it ends up in his treasure pool, it's because he won the lot. If he was suppose to get the drop, this system saved you about 5 seconds, if not, then the time it would take to cast a lot, he can use to pass it on to the proper recipient.... or rip you off.