Rayburn was expected to be a great DT for the Eagles down the road, he was originally signed as a free agent but was rewarded with a long term contract after he impressed the staff with his play. He had 5 years remaining on that contract. He is just 26 years old. The Eagles are loaded at the DT spot so somebody had to go.
I don't see the Cowboys being interested in him since we have plenty of guys already who can play DT in a one gap scheme. Since Phillips uses a one gap instead of the two gap that Parcells preferred, there is no urgent need for a true NT to groom in place of Ferguson. The guys we have already on the roster will do just fine in this defense.