Is the action of an employee who gave away a piece of an appetizer after it was packaged, then re-weighed, re-priced, and re-packaged it, religiously permissible? And was the action of the questioner, in returning the package out of fear of what is unlawful, correct?

There is no blame upon you for declining to purchase the appetizers as long as you are with the seller in the contracting session. Both parties have the option to annul the contract as long as they are in the contracting session, according to the saying of the Prophet (peace be upon him): "The two parties to a transaction have the option [to annul it] as long as they have not separated, except in the case of a sale with the option clause." Your annulment of the contract in the session is permissible, but your reason for doing so is not valid, because the fear of weight deficiency is groundless as long as it is with the seller's consent.