The 33-year-old, who made 82 appearances for the club, has not featured since March, while a calf injury was expected to keep him out for the remainder of the season.
"Nick wanted to stay at the club and we wanted him to stay at the club, but we can't make ends meet," Bentley told the club's official website.
"We've agreed to pay his contract up and he'll leave the club. That wobbles you because he's a big figure in the dressing room and the type of player that others look up to.
"I put the news to them (the squad) that he's leaving and it's sad to see. I'm devastated because he's a great lad."
There have been no indications that Fenton will hang up his boots.