You should go back and let the manager know. The waiter is a thief, and should be fired. And speaking of thieves, a waitress told me once, never leave the tip area on a credit or debit receipt blank, if you left the tip on the table. always draw a line through it and write the total on the space where you would write the total, had you left a tip. She said some waitstaff would add in an amount after you leave, and steal it.
Having had credit card fraud numerous times over the last 20 years, we always pay in cash. And we do tip according to service. It’s a win-win. If the service was bad, they don’t have our name to try and do us harm. Yes, that is always a consideration in the West of the USA.