A pension fund in Detroit is suing Cleveland Cavaliers owner Dan Gilbert and Rocket Companies in a Delaware court, claiming he had insider information when he sold the fund $500 million in Rocket Companies stock.

The sale was made in March to cover a philanthropic pledge, but the Police & Fire Retirement System in the City of Detroit claims Gilbert knew the stock’s value would drop soon after he sold it, according to the lawsuit.

Rock Holdings, Rocket Companies’ parent company which is headquartered in Detroit, issued a statement calling the civil suit “frivolous.”