my friend let me use his credit card to buy shirts on line which were on sale for $100.00. a few weeks later i sent it back because of a problem with the shirt. it was not on sale then so they credited the credit card for $150.00. the regular price. i told my friend he can keep the extra $50.00. is that a problem of ribbos?
There is no problem of Ribbis as you the borrower paid back only what you borrowed, and the additional money came from another source.
See Shulchan Aruch Y:D 160:13