"Ah, no--I mean, y-yeah, but! That's okay! I didn't have enough money to pay for a whole outfit anyway! We'll figure something out! Don't worry about it. Let's just enjoy your birthday. That's the whole point of this!" If they had to talk about the issues later, then fine. But for now this time should be to relax and think about--Well, anything really.

"It looks good on you!" Yes, a perfect distraction! How can Barnaby suspect when he's being so wonderfully complimented. Though, to be fair, Barnaby was the kind of guy who would look good covered in mud, so it wasn't a hard sell.