Williams v Roffey involved the sale and supply of goods and services, whereas Foakes v Beer was an agreement to accept part-payment in full settlement of a debt.
Re Selectmove is a case which recently (1995) brought about an argument for reform of the Foakes v Beer rule, and the judges were willing to hear Williams v Roffey type arguments, but ultimately they were bound as it was a Court of Appeal case, and Foakes v Beer is House of Lords. Also, ReSelectmove did not comply with all of the rules for there to be a Promissory Estoppel remedy.
Stilk v Myrick was reformed by Williams v Roffey where a practical benefit is gained by the promisor, even though the promisee is doing no more than already contracually obliged to do.
For the moot point- he can refuse to pay more because the promisor in fact suffered a detriment from the penalty.