Belgian side KV Mechelen say they will take “the necessary steps at Fifa” after Sheffield United signed Dutch goalkeeper Michael Verrips.
He joined the Blades on a free transfer hours before Thursday’s deadline.
Verrips, 22, terminated his contract with the club after Mechelen were removed from the Europa League following a match-fixing investigation.
The club say Verrips “unilaterally broke his agreement” – but are not thought to have an issue with United.
Mechelen added that the Blades made a “concrete offer” on Wednesday but “suddenly withdrew the bid on Thursday afternoon to announce it a few hours later as an incoming transfer from a free player”.
In a statement, the Belgian top-flight side said: “A transfer from Michael has always been negotiable, but his entourage wants to get out of the transfer fee.
“We therefore take the necessary steps at Fifa.”
Verrips joined Mechelen last summer and played 27 times.
“I’ve been welcomed well and I’m looking forward to the challenge, I’ve already got a good impression about the club and the people here,” he said after signing for the Blades.