Australia international full-back Cameron Shepherd is suing Northampton Saints for pulling out of a deal to sign him.
The club announced in May that the 28-year-old would be moving to Franklin's Gardens on a three-year contract.
But earlier this month, they called off the deal citing "fitness concerns" and Shepherd joined Sale Sharks instead.
Shepherd's agent Nick Fordham said in a statement that he was "appalled at the behaviour of Northampton".
It is believed the player's left ankle was the main cause of concern for Saints.
But Fordham's statement continued: "Expert medical reports from leading independent orthopaedic surgeons and Cameron's former club doctors at the Western Force have concluded that there is nothing wrong with Cameron's ankle.
"Having subsequently passed a medical and signed with the Sale Sharks proves there are huge holes all through Northampton's action and position."
When the deal was called off earlier this month, Northampton director of rugby Jim Mallinder said the club needed "more certainty about his ability to handle the tough demands of our long season".
But Fordham claims Shepherd, who was born in England but has nine caps for the Wallabies, was only told of their decision three days before he was due to leave Australia and a month after his medical had taken place.
"The club has leaned on a very loose medical clearance clause in the heads of agreement which is purely in the club's favour. This is now being scrutinised by the UK-based Rugby Players' Association.
"What's the point in having contracts where things can be completely fabricated to break contracts and then cause damage to one's professional reputation, as has happened in this instance?" his statement added.