Transactions in a currency other than the card currency are processed by the Visa/Mastercard within 2-3 days. The transaction has two stages: funds are first held (pre-authorization) and then debited. If the exchange rate changes when the funds are debited, the amount will be different from the amount originally held. Depending on how the exchange rate fluctuates, the debited amount may be greater than the amount originally held. If this happens and there are insufficient funds on the card, the balance will go into the negative.
We neither charge penalties for a negative balance nor block your card. Just add the required amount of funds to the card as soon as you can.