In Chapter 6, when Harry was buying the trunk: his whole speech about planning fallacies and collaborators ... wasn't really necessary, was it? Even had he not stolen his own money, I doubt that the proprietor would have refused a down payment accompanied by a request to hold the trunk overnight, pending the remainder of the sum to be payed in the morning.

That said: what if he had simply withdrawn the eleven Galleons and presented them as a fait accompli, without preface?