Shalom kavod Rav if my friend at work laid out 9.71 for lunch one day, can I give him back $10 and tell him keep the change as a matana/ or tell him put it in tzedaka, or is this a problem of ribit?

My wife babysits her friends baby once a week for about 3 hours and gets $40 each week. Today her friend asked if she could come 10 minutes early, My wife said yes as a friendly favor. Her friend left $3 extra in the envelope but my wife didn’t ask for any extra money. She wants to give it back. Is there a problem of ribit here?