Cardiff will allow long-serving Kevin McNaughton to leave this summer along with Danny Gabbidon and Nicky Maynard.
Highly-rated Atletico Madrid midfielder Saul Niguez has signed a new five-year contract.
United boss Nigel Clough is keen to bring the defender back to Bramall Lane after a successful spell on loan last season.
Freeman spent the last two months of last season on loan at United, impressing in his 12 appearances for the Blades.
Clough is a big admirer of the 22-year-old, having signed the full-back during his time in charge of Derby, and he has made him his top target.
Derby, who have already brought in Cyrus Christie from Coventry, have turned down United's offer for Freeman and the news has alerted a number of other clubs.
The likes of Leeds, Blackpool, Peterborough and Doncaster are all thought to be monitoring Freeman's situation at the iPro Stadium.