Former Lyn Oslo director Morgan Andersen has been found guilty of forging contracts relating to John Obi Mikel in 2005.
Andersen, who has told the court he plans to lodge an appeal, was handed a one-year suspended jail sentence for the offence.
Nigeria international Mikel, who left Lyn to join Chelsea in the summer of 2006 after a protracted transfer saga also involving Manchester United, initially joined the Norwegian club as an amateur as he was under 18.
After the midfielder's 18th birthday, the Norwegian FA were presented with a professional contract which Mikel has denied signing.
The court ruled that the falsified documents, which the judge decided had been cut from Mikel's amateur deal, meant that Lyn were able to gain a share of the player's transfer fee.
Judge Torjus Gard said in his judgement: "The court is in no doubt the contracts were amended and Andersen knew this."
Andersen is currently on leave from his post as director of Fredrikstad, another top-flight club in Norway.
Mikel performed a U-turn over a proposed move to Old Trafford, claiming he had been put under pressure to join the Red Devils.
Chelsea later reached a financial settlement to secure his services, paying £12million to their Premier League rivals and a further £4million to Lyn.