Select users are being given a ‘Pay Later’ option when they try to acquire Facebook Credits. Facebook displays a negative balance for users who haven’t paid for their Credits, and gives them the option to pay their balance. At this point, there doesn’t appear to be any limit to how big a balance the affected users can rack up.
Presumably that would change if this feature were rolled out more widely. But letting users get a free taste of Credits is smart — getting people to enter credit card information for the first time is the biggest friction point in virtual goods sales. If users get used to spending Credits, they’ll be more likely to pay up to acquire more.