If you try to buy something in a game or app that costs more Facebook Credits than you currently have in your account, your credits balance will be used towards the cost of the item and you will be prompted to purchase more credits, which will be charged to your payment method.
The following example might clarify how in-game purchases work:
To see a list of available currencies, go to the Payments tab and click "change" next to "Preferred Currency". Purchases in non-US currencies are first converted to US dollars using the current exchange rate, then Facebook Credits are added to your account at USD $0.10 each.
Depending on your mobile carrier and your country, the mobile payment option may only be available for purchases under $10.00 of credits.
Note: when you add a credit card to your account, Facebook makes a small temporary charge to validate your credit card. This charge should disappear from your credit card statement within 3 to 5 business days.