If I was there as a customer I would have raised holy hell — “LADY — THAT GUY JUST DROPPED THAT BILL! DID YOU REALLY JUST PUT IT IN YOUR PURSE!!!”
Had events continued with her denial, etc. I would have ordered and then added $20 to my payment with a quiet smile a nod.
I would not want to “undo” his clearly classy and generous (and, frankly, brilliant) offering but $20 is not much to me, but it is several hours work for him.
I ain’t bragging — I am just noting what I would do and I am sure most (I will say all) FReepers would do the same.