If Dareus was traded now, the Bills would get hit with $20.6M in dead money. If done before next season dead money would be $10.44M, but at least the team would see a small net increase in cap space.
Trading him is reall the only option in the near term, but I'm not sure it's much of a threat. Either way he's getting paid and it's not like he's getting shipped out of a Super Bowl contender or particularly desirable city. (I love Buffalo, but don't tell me Dareus wouldn't rather be in Miami or LA.) This contract for this player is Exhibit 1-A on why Whaley had to go.
Here's a link for anyone wanting contract details. Note the tab for different cut dates/trade. That can be used with the year tabs to quickly see what the impact of cutting/trading players is at different times.
https://overthecap.com/salary-cap/buffalo-bills