The 23-year-old shotstopper signed for the South Coast outfit in the summer after coming to the end of his contract at Reading.
But the Royals are hoping that a tribunal will insist that Portsmouth must pay a bumper £400,000 fee for taking Ashdown off their books.
Club officials will travel to London for the hearing on Thursday 23rd September.
Although out of contract players normally move for nothing, the Royals will receive a fee for Ashdown's departure as he was under 24 when he left the club.
Fifa rules dictate that when a young out of contract player moves, and has rejected a new contract offer, his club are entitled for compensation for developing the player.
Ashdown, who was for a long time Reading's second choice goalkeeper, established himself in the first team at the end of last season with a string of classy performances.
But the 23-year-old former Arsenal loanee snubbed the chance to sign a new deal at the Madejski Stadium when Harry Redknapp's Premiership team offered him big wages and the chance to challenge for their number one spot.
Ashdown has been on the bench for Portsmouth for the opening three games of the season.