Deactivate family sharing payment method without turning off purchase sharing

Hello!

Is there a way where i can deactivate the shared payment method as a organizer of a family without turning off the purchase sharing for my family? why is this option combined with each other...

The problem is simple: my sister buys for example movies for her kids, like i do for my kids.

But she always has to give me the money back an make a transaction for it...but we still want to share purchases....

the only way, for me as an organizier, is to deactivate the payment method, but this deactivates the purchase sharing, too.....

Why???