If the person has a credit in their account the credit will be used instead of being charged to the primary person, otherwise there is no alternative. I guess that's how they make sure it really is a family using it where there is a certain level of trust. Otherwise a "family" might end up being a college dorm.
The other payment control is if one of the people is under 18 at the time of setting up family sharing and ask to buy is activated.
Family purchases and payments - Apple Support - https://support.apple.com/HT201079 - "Are you the family organizer? Learn how purchases work with Family Sharing, what you're responsible for, and how to manage family purchases and payments."
Request and make purchases with Ask to Buy - Apple Support - https://support.apple.com/HT201089 - Ask To Buy can only be turned on for members up to 18.