If he takes any money and doesn't donate it back to the charity then everyone should boycott the tournament and just donate the money directly to the charity.
At my former course every holiday they'd have a Beat The Pro's scramble. But everyone got strokes based on handicap. It wasn't a charity event and any teams that beat the pro's won money. It was fair.
Any handicapped golfer who plays a Pro straight up for money should have their head examined.