When someone insisted on giving extra money as a gift after I helped them

A friend asked for financial help and promised to return the money along with a bonus as a gift. I mentioned, "Assalamu Alaikum, I can lend it, but please keep the extra amount because I want to avoid anything resembling riba." They responded that it wasn't riba, just a kindness for assisting them. I politely declined again. Still, when they repaid, they included the extra sum despite my refusal. Any thoughts on what I should do? JazakAllah Khair.