i think the problems are much deeper than just the money. Maybe the wife didn't get along with the Mother In Law? And maybe methods of taking care of the baby is different and therefore 磨擦 = conflict? There has to be a much deeper reason for the case other than just the rent. Think about it, he's been living with his parents for his entire life..why fight for the money now? it could be the wife or the baby etc etc.
they are only using $$ as a way of conflict because it is tangible and can be won and lost. And when they put money on the table, there's no going back. Money fuel angers and all relationships are harmed.