He used the credit card from the National Bank and was obliged to pay an extra amount for buying in instalments.
In order for it to be permissible to use credit cards, it is essential that they be free of things that are forbidden in sharee’ah, which includes the stipulation of a penalty or additional payment for paying by instalments or delaying payment, as has been explained in the answer to question no. 104244 and 116442.
It is also essential that the fees for issuing the card do not exceed the actual costs incurred by the bank. See the answer to question no. 97530.
You have done well to stop dealing with this card, because it is not permissible for you to use it in the situation that you describe.
In previous answers we have explained that it is not permissible to use credit cards that impose penalties for delay in paying even if the person is determined to pay on time without any delay, because his entering into this agreement implies approval of and agreement with riba, in addition to the fact that he may delay a payment because he is travelling or busy, in which case he would actually be obliged to pay riba.
As for the amount that they charged you as the result of the employee’s error, the sin of that is on them, and there is no sin on you in sha Allaah, because you did not agree to it, rather you paid it reluctantly.
And Allaah knows best.