Brent, 27, signed a one-year extension with the Cowboys in November of last year. He would have made a base salary of $745,000 for the 2015 season.
Brent served a 180-day prison sentence after he was convicted of intoxication manslaughter. He was released from prison in June and finished out the remainder of his sentence in a substance abuse center.
He wound up receiving a 10-game suspension from the NFL for the incident.
Despite the serious off-the-field issues, the Cowboys stuck by Brent and were optimistic about his NFL future. Last year, he was limited to appearing in just one game and recorded two tackles.