Nainggolan, 32, played for Cagliari between 2010 and 2014 and returned on loan last season.
Cagliari want to bring back ‘Il Ninja’ on a permanent basis and have held progressive talks with Inter.
The Sardinians have made an offer of €12m plus two Cagliari Primavera player to Inter.
Riccardo Ladinetti, 19, is ready to move to Inter and could be joined by one of Roberto Biancu (20), Federico Marigosu (19) or Luca Galliano (20).
Cagliari are now trying to find the balance of the deal with Inter and an agreement is close.
Nainggolan has two years left on his €9m-per-season deal and Cagliari could spread his salary out over a three-year contract.
The dynamic midfielder will miss Inter’s next game due to pharyngitis.