I don't see why he would have to. It doesn't matter if any of 2017 is guaranteed now or not. In the event of a trade before the season starts, the Eagles are on the hook for only the signing bonus, thereby gaining cap space, and their trade partner would be on the hook for the balance and have two years with Bradford on their roster.
Eagles cap position improves.
Their trade partner gets a moderately priced QB for two years.