Discarding items that are part of a deal

[Michiel]

Here's the thing. I believe that when you make a deal with someone to help you like "you get the two treasures if you help me" you are obliged to actually have your helper get the treasures right?

In this case the deal was that my helper got the treasure AND one of my items. Before we defeated the monster I decided, as a thief, to discard an item in order to steal something and I discarded the item that was part of the deal. Now, it is very mean, very nasty, but I also thought it was very funny and legit. We had some discussion about it, but as owner of the game I decided to go on with it and so we did.
He was angry with me and I didn't get a slice of his pizza, but I wonder whether it's actually legit or not. Can someone tell me?

Thanks