Song has long been available due to the Cameroonian failing to establish himself at the club since his 2012 arrival, but Barca’s desire to allow Alves to move on is somewhat a surprise.
It is thought that the Catalan giants feel that this summer could be their last chance to recoup a decent fee for the 30-year-old, whom they may be able to replace with Martin Montoya.
Arsenal have been linked with the Brazilian in the past, but they may face competition from City and United for the rampaging full-back.
Song is also a target for the Gunners, yet they will likely face competition for their former star from United.
Red Devils chief David Moyes is known to be keen to add energy to his engine room, and it is thought that the 26-year-old fits his ideal specification.
Song himself is speculated to be in favour of a move back to England, which could put Premier League sides ahead of clubs from across Europe.