How much does the restaurant charge for the item OP ordered? If it's $4.75 (hardly unusual for a high-end place which OP says it is), that's what OP should have been charged. If the restaurant wanted to comp the item, that's fine too.

But, stiffing an employee because he charged the price that the establishment charges? That's unfair.

If OP cared to ask, he could have asked whether there was a charge for the soda and then dealt with the issue later.