Avoiding Ribis by giving it to Tzedaka?

I went to a bagel store to buy a bagel for me and a co-worker. I told her not to pay me. She sent me $8, when the bagel was much less. Is this Avak Ribis? I don't want to offend her by giving it back, can I give the $8 to tzedaka to avoid ribis?

Thank you for your question.

There is no issue regarding Ribbis since this would not be called borrowing money from your friend, rather it is considered as doing a favor for her or a shlichut on her behalf. 

Therefore, as long as she knows how much it costed, and she tells you to keep the change, there is no halachic problem for you to do so.

Wishing you much success.