If you do not have any Facebook Credits in your balance or not enough to purchase the selected currency package, you will be prompted to buy the amount of Facebook Credits necessary to make the purchase.
If this is your case and you are using a credit card or PayPal to make the purchase, you will be able to buy the exact amount of Facebook Credits needed to purchase the currency package. However, other payment methods such as mobile phone may require that you buy some more Facebook Credits than needed; in this case, once you complete your purchase within the game, the currency package you bought (e.g. FarmCash) will appear in your game and the remaining Facebook Credits will be added to your Facebook Credits balance.
Please note your game item purchase may occasionally experience errors, in which case the equivalent amount of Facebook Credits may have been deposited into your Facebook Account balance to be used to complete your in-game purchase at a later time.
You can check your Facebook Credits balance in the Payments tab of your account.