I feel like if you offered a trade, but cancelled it before they accepted it... it's fine.
To be honest, I really think you can drop out of the deal any time before you send the confirmation message. If you send it, it's done. But, if you both agreed to it and one person didn't send a confirmation, said person should still be able to cancel.
It's not very nice, but it makes sense (to me at least).
I did it once, because somebody accepted my deal too fast (sorry Truthfot) and it didn't feel right. If someone says "Yea" in like 5 seconds, it usually means it's so obviously favored towards them that they need to get the deal done ASAP.
Another time, I had a great deal but said that I wanted to think it over. When I finally said, "Let's do the deal," they cancelled it (that's you, AB!) and I didn't get my great deal
But he should have been able to. I'm not against it (even though I'd have a cheaper PG and a star instead of Greivis)
So... yea. I think you should be able to withdraw anytime prior to messaging Roy (or other mod)