Hermione could be the one. With her vast knowledge of the wizarding laws and her sharp intellect, she might dig up some old regulations that prove Harry's right to his money. She would then fight tooth and nail in the wizarding courts or against the goblins at Gringotts to get his money back.
Maybe he discovers some legal loophole in the wizarding world's financial system that allows him to reclaim his inheritance. For example, if there was some improper handling of his family's vault by the goblins at Gringotts, he could use that as a basis to demand his money back.
In some fanfictions, Harry might hide his money in a secret compartment of his trunk. He could use magic to create a hidden space where no one else can find it.
Harry Potter could use his money to improve Hogwarts in fanfiction. He might donate to renovate old classrooms, buy new equipment for the library or even fund extra - curricular activities for students. After all, he has a strong connection to the school and wants to give back.
The 'harry gets his mail back fanfiction' could start with Harry noticing that he hasn't received any mail for a while, which is strange in the wizarding world. He starts to suspect foul play. As he investigates, he finds out that there is a new spell that has been cast on the mail delivery system that is misdirecting his mail. With the help of Ron's family's connections in the wizarding world (since they are so well - known in the wizarding community), Harry is able to track down the source of the spell and reverse it, finally getting his mail back.
He could be disillusioned with the wizarding world's politics and power struggles. So, taking his money and leaving is his way of getting away from it all. He might feel that he can't make a real difference within the existing system and decides to find his own path.
Maybe he needs the money to fund an independent research project. He's always been curious about certain magical phenomena, and withdrawing his money would give him the financial means to pursue that knowledge without relying on Hogwarts or the Ministry.
He might use it to buy rare magical artifacts. For example, he could purchase ancient wands or enchanted jewelry that have special powers. This not only adds to his personal collection but also gives him an edge in certain situations.
In the fanfic, it could be that Harry has a very smart friend who helps him come up with a clever plan. For example, they might find a loophole in a contract that the person who has Harry's money was using to keep it. So, by exploiting this loophole, Harry is able to legally get his money back. Another possibility is that Harry has some special skills or powers in the story that he uses to his advantage. He could use his charm to persuade the person who owes him money or his intelligence to outwit them in a negotiation.