Unknown to Harry, Petunia had made Vernon stop in London so she could pay a visit to the wizarding bank and give those nasty, thieving creatures a piece of her mind before they left England. She told him she was aware it might not do any good but she really felt she owed it to Lily and to Harry to try. It was her duty to do for Harry what she believed Lily would've done had she survived to do so. And Lily wouldn't have hesitated to tear those thieving goblins a new one for ripping him off the way they had.
Since Vernon was also outraged at how his nephew had been treated he hadn't argued too hard about it and only remained in the car because of Dudley. Dudley had made it clear the one time he'd been allowed in the Alley, he saw nothing wrong with owning magical devices even as Vernon and Harry had repeatedly told him it was illegal for him as he wasn't magical. He knew taking Dudley back into the Alley would only have his son whining and complaining about all the neat things they wouldn't let him get. Whining and complaining that would likely carry on for weeks before the boy would forget about it. So there was no way Dudley was going into the Alley again.
But Vernon and Petunia had both taken offense at the goblins trying to absolve their own guilt by placing it on an innocent child. So a stop was justified. Someone needed to speak up for the boy and make those disreputable thieving creatures do their bloody job. Which was probably why the goblins had gone along with Harry's plans for the thieves. Neither one had stopped to consider just how Petunia would be able to get into the Alley without Harry to open the portal for her. To be honest Petunia hadn't even considered it might be a problem She just made her way through the tavern into the alley and tapped the appropriate bricks with her finger and the portal opened. She'd then made her way to the bank.
Once in the bank she'd been subjected to a lot of sneering and comments of how it was Harry's fault as the account holder and not their problem if he came of age only to find there was nothing for him to inherit due to his poor management skills. That hadn't lasted for very long.
She understood a lot more than the goblin thought she did and it wasn't long before she was raking the goblin over the coals of her very sharp tongue and browbeating him verbally into agreeing to rectify the situation at once. Showing a greater understanding of the goblin nature than any muggle was supposed to have, Petunia had forbidden the goblin the use of his own name until every last stolen or fraudulently removed coin and item had been returned to her nephews accounts with appropriate interest and every single crook had been dealt with accordingly. And the final person on that list had better be the no-name goblin toe nail scrapping she had the misfortune to be speaking to at that moment. Every person in the bank at the time, whether human or goblin, stared in shocked silence as this mere slip of a human woman took the goblin teller apart with ruthless efficiency.
She let him know, in no uncertain terms, she and her husband were Harry's lawful and legal guardians and had been since the night his parents had died leaving him an orphan when he was nothing more than a helpless fifteen month old infant. She bluntly informed the smug goblin neither of them had signed one single paper allowing anyone to withdraw so much as the lowest valued coin from his vaults. Let alone valuable artifacts, jewels or books. Nor would they have done so even if they'd known he had vaults there. And for certain Harry hadn't signed such a paper since he'd only been a year old at the time. Can goblin babies read and write at a mere one year of age? The terrified goblin mutely shook his head. Why then did he think a human child could? So the Goblin Nation was to make this travesty right with appropriate interest on the debt they owed him since they should have bloody well known any authorization they'd received was fraudulent. The thief wouldn't have gotten away with their crime if the goblins hadn't aided and abetted him or her in doing so. Or she would do her level best to personally ruin them all. And with what they'd allowed to happen to Harry's accounts, she was pretty damn sure she could carry out her threat very damn easily.
The goblin, who had paled when Harry's Aunt had begun loudly taking him to task right in the middle of the bank and in front of his supervisors, had whimpered when she demanded interest be paid on the Debt of Theft. A grim look from the supervisor had told him to agree and get the angry woman out of the bank as quickly as possible. So he had promised the cooperation of the entire Goblin Nation with Lady Tonguelash, not even noticing he had accorded her with high goblin honor by naming her as such. Within a week the locks on every Potter vault had been changed with new keys issued.