Tips are not expected in Japan
Unlike other countries, we have no “tipping” custom in Japan. It can cause a trouble, so please do not leave your money on the table.
“Keep the change” is natural way to give them extra money.
Saying “Keep the change” [ Otsuri wa kekko desu] is what Japanese people sometimes do to show the extra appreciation. In this case, the “extra” money go to the restaurant, not to the waiter.