In truth, you didn't sell that player: he was never a member of your squad.
The player in question, Thibo Baert, belonged to the franchise when it was managed by someone else. That owner left the game and because Thibo was rather good, he was released onto the market as a free agent instead of being deleted from the game like his team-mates. Thibo sold for $150,000 on that occasion. This was in November last year.
You took over the franchise about a week ago. You are owed no money. It's just one of those things: free agent transfers never get deleted from a teams transfer record.