There must be provisions in the agency agreements to keep players from working out a deal with one agent and the switching at the last minute to another party who'd then scoop the commissions. 5 days, in most situations, are sufficient to change the FA landscape.
In this case, Magid is going to lose more than this one commission. A screwup like this is hard to imagine. He's in damage control now, and just has to do whatever can be done to patch things over as much as possible. Dumervil got tubed pretty bad here. I hope he lands on his feet, because it sounds from all accounts like he's trying to be a good teammate and a reasonable actor here.