Moscarda does this to prove the book's thesis: he wants to show, in a single act, that he can also, for others, stop being the usurer they see in him. The eviction and the donation are the first experiment, and the result is the opposite of what he expected.
Marco di Dio is a former apprentice sculptor, convicted for an act committed in a single moment, who lives on schemes for getting rich and on hatred of Moscarda's father, whom he calls a usurer more furiously than anyone. He lives rent-free in a small hut of Moscarda's, and it is this little hut that gives the narrator the means to attempt the proof. He understands that, by acting against himself, he would play a bad trick on the usurer that others see.
At the bank, Moscarda quarrels with Firbo, kneels on the floor, orders Marco's eviction and steals the house papers from himself. He watches the eviction in the rain, protected by a police officer, hearing "usurer!" from the crowd. When the young man from the registry office announces the donation of a house, Moscarda awaits a transfiguration, and what comes are whistles and insults, because no one understands that the donation is his after the cruelty of the eviction.
The effect is a swap of labels. When Marco di Dio meets him in the corridor of the new house, he leaps at him shouting "madman!", the same cry as the crowd. The usurer could go mad, but could not destroy himself in a single blow. The book says so plainly: he had wanted to demonstrate that he could, for others too, cease to be what they believed him to be.