He used the credit card from the National Bank and was obliged to pay an extra amount for buying in instalments.
Question

                I took a Master Card from the National Bank in order to buy online. I wanted to pay all at once so that I do not have to pay any extras for installments. I was surprised when I received the receipt that they have added an extra amount for buying in installments and another extra for issuing my purchase, although I was told it is free. I meant to pay all at once, but the clerk who entered my data made a mistake, so they charged me extra and made the payment in installments. When I faced them with this mistake, they said that the bank does not bear a clerk’s mistake and that they will not cancel this extra. They refused all my trials to cancel it justifying this by the ‘bank’s policy’. I cancelled this card and stopped dealing with them. I am waiting for your opinion about what I should do. Am I considered sinful, although it is the clerk’s mistake no mine? Bear in mind that I have already stopped dealing with them until I receive your answer.
Praise be to Allah.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.