After extensive negotiations, the Catalan giants announced the signing of the Juventus midfielder earlier this week, allowing Arthur to move in the opposite direction.
Although Pjanic turned 30 years old in April, club officials have felt the need to protect the Bosnia-Herzegovina international's time at Camp Nou with an extravagant buy-out clause.
A club statement read: "The cost of the operation will be €60m (£54.36m) plus €5m £4.53m) in variables.
"The player will sign a contract with the club for the next four seasons, until the end of the 2023/24 season, with a buy out clause of €400m (£362.42m).
"The player will remain with Juventus until the 2019-20 season is over."
Pjanic has made 169 appearances in all competitions during his four-year stay in Turin.