Lucius Malfoy is going broke, he spent all of the Malfoy fortune on funding Voldemort and paying bribes to get out of azkaban. Even his wife is thinking of leaving him and marrying another pureblood.

Lucius finally gets his lucky break when, due to his connections, he gets the job of managing Harry Potter’s assets.

Since Harry is a orphan and has no magical relatives, the ministry takes control of his assets and vault to manage it until he turns of age. The ministry does this for all orphans.

Due to Britian’s weak anti corruption laws Lucius can’t just directly steal the money. Instead Lucius sells the Potter assets for dirt cheap to other purebloods in exchange for favors and buisness deals which allows him to rebuild the Malfoy fortune.

Also he is legally allowed to pay himself “caretaker fees” from the vault. He can’t take everything, but by paying himself a significant salary for years he manages to steal a lot.

The worst part is that no one ever realizes. Lucius bribed the media to keep quiet when they discovered, goblins don’t care, Harry doesn’t realize how much money and assets he is supposed to have, and Dumbledore is a great wizard but he is not good at finance.