In ZEN, you can make transactions up to your balance. In rare occasions, the balance may be negative, in that case, the difference will be covered by your next deposit.
Sometimes it happens that card transactions settle after the blockade is already released. It means that the merchant sent us the settlement order too late. If in the meantime you make any other transaction, which leaves your balance too small to cover the card transaction in question, there will be a negative balance which must be covered as soon as possible.