My (non-observant) dad let me run a charge through my credit card to meet a spending bonus. We do this from time to time and he pays the bill directly on time. This time he asked if he could split the bill over a few months and pay the interest. I told him it might be an issue because of ribbis and I’d be happy to pay the few tens of dollars of interest myself. He offered to gift me the value of the interest in cash as an ‘unrelated gift’ (even though it’s basically for the interest). Is this mutar?
If the “gift” is given at the time when the payment is made, it appears to be what it really is, a payment for the loan, and it is problematic. Therefore you have one of two options. Either to sign a heter iska with your father. A heter iska is an agreement that the money that he borrowed from you, by charging you credit card, is being changed into a business deal and an investment, with the amount of interest being the lenders profit of the investment. Being that the money of this $1000 is not really being invested here the heter iska includes profit from $1000 worth of investment property that you father owns, which can appreciate. This can be in the form of stocks, money in the bank, real estate, or even his car, (since he can rent it out and make a profit on it). This way the money that he will be paying you will be profit dividend and not forbidden interest. It is important for the people involved to understand what the heter iska agreement is doing, otherwise it doesn’t have much value. You can download a copy of a heter iska here https://dinonline.org/2010/02/24/heter-iska/
There is another option, to tell your father, that at the time of the payment you can not accept any extra money, even if the form of a mere “gift”, and you are willing to absorb this cost. If he wants to give you a gift at a different time, he may. However he may not specify at the time that he is giving you the gift,,that it is for the interest that you paid.
Another point. This is all assuming that the credit card company abroad is owned by gentiles, because if it owned by Jews, then you have a serious issue of paying interest on the loan that you took from them.
Shulchan Aruch Y”D 160-4,5, Rema 160-6.