Nicky Byrne. In 1964, he proposed to Epstein an offer to handle all The Beatles' merchandising arrangements (lunchboxes, board games, headbobbers, etc.), but ONLY if he was given 90% of the profits.
He didn't think Brian would really go for it, but Brian told him that 10% was better than nothing. Byrne got it in writing and made a killing. It has been estimated that the Beatles' potentially lost $100 million on this deal.
Brian later sued to get the rights back, and the case wasn't settled until 3 weeks before he died, but I'm not sure of the details.