What is the reason that Agent A asks for an extra £10? Is it because he had a bucket of green paint laying around that is about to go bad, and he doesn't want to spend £9 to drive to the store to buy black paint? Or does he believe he can get away with demanding more since the contracts needs to get amended?
The second. The scenario is contrived such that it contains credible commitment to self harm that also harms the other.
My blackmail posts have generated some interesting discussion, so I'm just creating this one so that people can post examples of behaviours that they think are either clearly blackmail, or clearly not blackmail, or something in between.