Paying back only the principle amount in an interest bearing loan

My dad took a credit card back in Oman and told us that he is taking it for emergency purposes and told us that he will pay back the bill within 52 days as to avoid interest according to the bank. We lately moved back to Pakistan and used the entire credit card while coming and now we have the money but 52 days end on this 15th and we have kept the money but can't find any source. So If God forbids we can't pay back before the 15'th and the interest is charged and if we only repay the principle amount but not the interest, will we be punished? Is it haram?