This question does raise an issue of duress, but it first raises an important issue of whether the contractual variation is binding. This involves discussing the requirement of consideration, examined in Chapter 7, and in particular the difficult case of Williams v Roffey. That case might be distinguished here, because Mike (impliedly?) asks for money from Harvey, rather than Harvey (freely?) offering more money to Mike (which would be more akin to Williams v Roffey). If Roffey is distinguishable, then you need to go on to consider whether the elements of a claim in duress have been made out. In particular, you might discuss whether Harvey was left with no reasonable practical alternative but to comply with the threat.