So according to hacked screenshots by Stanford computer science student Andrew Aude. The story here is that payments would be send the same way you can now send a photo. A debit card would be added to your Facebook profile. Facebook will also give you the opportunity to add a PIN for extra security.
Aude told TechCrunch how the payments work (at least so far):
Aude played around the with feature and its code. He tells me you simply hit a button to initiate a payment, enter the amount you want to send, and send it. Facebook keeps the transaction private and doesn’t publish anything about it to the News Feed.
In the version Aude investigated, Messenger payments only worked with debit cards, not credit cards or bank accounts. That’s likely because money transfers are cheaper to process when they come from debit cards and don’t require approvals or numbers some people don’t know off-hand like connecting a bank account. Aude speculates that “based on my understanding of the debit interchange rates, each transaction will cost roughly $0.40 to $0.50 (Durbin swipe fee + ACH fee). The app didn’t mention a fee to end, so it’s probably free, at least initially. Over time they might add a $1 fee.” This can’t be confirmed, though.