Was his brother really his agent or was he an advisor like Haaland's dad, with Raiola as the main agent?
In any case - when it comes to money, never hire or take advice from family!
Also - get everything in writing. There is no such thing as a Gentleman's agreement. Imagine being the EPL all-time top-scorer but never having won even the League Cup. He should push for a move before the age of 30 because other than Italian clubs (who have fallen considerably), his options will decline considerably.