MarketPress Refund in a marketplace with paypal chained payments

Heya,

How does MarketPress reacts on a potential refund from a Seller in a marketplace?
If there is an automated way then would both transaction been canceled or not?

My goal is fairly simple, I do not intend to refund the commission if the seller makes a refund and before finishing my T&Cs would need to be sure this would fit with the actual product in place. (Considering the commission is a platform usage fee, and avoiding sellers trying to bypass the commission by asking refunds...)

Thanks a lot for your help,

Cheers,
--Olivier