Dallas has all the leverage. They can cut him after June 1st and save his 3.25m salary against the cap and in real dollars. The Signing Bonus money is just icing on the cake. If he shows up out of shape or doesn't try, they can still get him for breach and put him on the suspended/non active list and not pay him. If he skips mandatory camp days they can start fining him.
You are right that If he really doesn't want to play they should just work out a settlement, something like 1.5m. This team can easily get by without a non-motivated Kyle Orton and they may already have a comparable replacement in Weeden. They could always sign a vet like Shaun Hill that played for Linehan before.