When you make transactions in currencies other than your card's primary currency, it takes these transactions 2-3 days to settle. This is how Mastercard work. The whole process has 2 primary stages, pre-authorization (funds are blocked on the card), and final settlement (the amount is charged from the card). When the final settlement happens, the currency exchange rate for the day of settlement is used. It can technically be different from the exchange rate used for initial pre-authorization. Depending on how the exchange rate behaves during these 2-3 days, you may notice that the final charge is slightly more or less than the pre-authorization amount. Sometimes a bigger final settlement may result in a negative balance.
Offline authorizations can also cause your card balance to go negative. Such type of transactions can be used by car rental services, airlines, gas stations, toll roads and other similar services. They can overcharge any type of bank card if they need to charge more at the end of your rental period or if POS had no direct connection with the bank at the moment of payment, like in the case of airlines.
We do not charge you for having a negative balance, and we do not block your card. Just fund it again as soon as you can.