Somehow I don't think Darwin would be welcomed back with open arms.
According to Schefter, Buffalo included the clause in the original trade that brought Walker and a 2008 seventh-round pick to Buffalo in exchange for Takeo Spikes and Kelly Holcomb, because they knew Walker was unsatisfied with his current contract.
The Eagles have no use or no room for Walker and I would imagine they would not be happy to take him back. They certainly wouldn't be interested in giving him a new deal.
This is pretty cool. Todd McFarlane, of Spawn fame, has designed a series of 3 Eagles figurines for Toys R Us.