Before the surgery, the family member slipped a red envelope into my white coat pocket.

I didn't refuse and accepted it.
The surgery was very successful.
On the day of discharge, the family member came to settle the bill and found an extra amount on the account — exactly the amount of the red envelope.
"That..."
"I didn't touch the red envelope; I kept it for you. But I know how to proceed with the surgery."
The family member was stunned: "Then why didn't you refuse it at the time?"