Just to make sure: did Stu get to Obliviate Rabastan before the trade? When he learned of the trade, he offered to do it the next morning, because he'd have just messed it up if he'd done it the night he got the body parts owl. We can either say he did or didn't, whatever is best for plot. We can also have him mess it up a little, if we want, as he wasn't exactly the most calm and concentrated for obvious reasons.