Suppose me and my good friend Shimon are in a store shopping. Shimon wants to borrow $100 from me for his purchase. I graciously agree, but instead of giving Shimon cash, I swipe my credit card for his purchase. Later, Shimon will pay me back the $100.
At this point it appears we have a rather simple case of borrowing and lending without interest.
The complication, however, is that the credit card I used is in-fact a cash back rewards card. So for every purchase I make, the credit card company gives me back 1% of the amount.
In this case, while Shimon will be paying me back $100, I will also be getting back another $1 from my credit card company.
So is this allowed? Or is it a case of interest?