By the looks of the last chapter if they can't pay they become akin to slaves
I have some issues with/questions about that theory though:
1. Why the need to threaten to gang**** the girl if her ability can turn her into a slave? Pure sadism?
2. Why is she not more concerned about making sure the other party has the means to pay? She was unwilling to bet her virginity before MC made his big offer, meaning she'd presumably not be satisfied with just getting a new slave while taking such a huge risk. Yet she didn't try and confirm MC actually was capable of paying up before accepting the bet, instead acting like her ability will make sure she'll get the money somehow. That does not sound like an ability that'll just turn you into a slave if you fail to pay.
3. It also makes the part where the MC noted that the money wasn't a problem for him irrelevant if you can just make up any number and betting girl will roll with it.
The ability just seems unclear and poorly thought out so far tbh.