The Brazilian international has suggested that his six-year spell at Camp Nou could be coming to an end as he is yet to be offered a new deal.
The 31-year-old's agent recently stated that Alves is keeping his options open, but FIFA agent Francois Gallardo has claimed that the full-back has already chosen United.
Gallardo told Spanish TV show Neox: "The player will leave Barcelona next season. He has signed a preliminary agreement with Manchester United."
Alves recently expressed a desire to play in England.