[William Cloud Hicklin]

But later Giles expects the dragon to act in fairness when he waits for him to pay up as agreed, and Chrysophylax does not.

I think that in part Giles was a bit complacent after his triumph; but also that Giles no more than the other villagers, even the educated parson, simply could not conceive that a sentient being, no matter how base, could possibly disregard the "solemn and astonishing oaths" which the dragon swore. A bit of a commentary on the power of oaths and their dependency on belief-- an enormous difference between the Middle Ages or even the 19th century and today.