The precise protocol is determined by the terms of the trade. "Traditionally the first condition is that the trade is contingent upon the player or players reporting and passing a physical," Signora said.
If the player doesn't report, the two teams can try to come up with a new agreement, or the trade could be called off. The only leverage that can be exerted against the player is the dreaded "five-day letter," a precursor to placement on the reserve/left squad list.
"The club sends a letter to the player warning him that, in five days, the club has the right to place him on the reserve/left squad list," Signora said. "It explains to him that if they do place him in that category, he cannot play again this season, etc. After sending that letter and the expiration of the five-day period, the club does not have to place the player on the reserve/left squad list, but it may do so."
Once on the reserve/left club list, Seymour would be unable to play for anyone in 2009, and his contract would toll by a full year, with his salary of roughly $3.7 million applicable in 2010, if he decides at that point to show up.
Seymour could eventually get "five-day letter" | ProFootballTalk.com