While this guy might have all the money in the world—and we mean literally all of it—he's not going to be sponsoring your next bake sale. Mammon, God of wealth in Book 2, is a real miser, obsessed with money only to keep it in his possession and accumulate more, not to share it with others or to make the world better.
Spenser's low opinion of wealth is pretty clear from the fact that Mammon's cave of riches is right next to the entrance of the underworld (you can't get much lower than that). Not unlike Despair in Book 1, Mammon's threat isn't in combat (he's pretty puny and dirty) but in argument. Similar to the temptations of Jesus in the desert in the New Testament, Mammon tempts Guyon with wealth and with arguments for why that wealth could be a force for good.
But Guyon, true to his temperate nature, refuses all of Guyon's offers, suspicious that a love of money can ever really accomplish anything, ultimately challenging whether Mammon really has the power over humanity he thinks he has.