What can I do to avoid a poor credit, when threatened by Mastercard?

You could call the cc company and ask for assistance but most likely they'll tell you there's nothing they can do but lower your rate, etc. You may be out of luck here because if you don't pay it, they will send collection agencies after you, calling you incisively, including your family, etc. They have a right to sue you for the balance along with court costs, etc and can then garnish your wages when you do get a job. It can hurt your credit quite a bit, especially if you don't have any other debt that you are paying on to counter-act the poor payment history.