a friendly fee
A friendly fee is like a tip you give your friend for letting you borrow their stuff instead of just taking it and acting like you’re doing them a favor.
My friend let me use her laptop and I gave her $5. She said I was the most generous person she’d ever met.
I borrowed my brother’s Xbox and dropped $10 on the table. He said I was a better brother than he was.
I took my mom’s phone and gave her $5. She said I was a saint.